Method for controlling a grid-connected inverter and grid-connected inverter

Abstract

A method for controlling a grid-connected inverter and a grid-connected inverter are provided.SOLUTION: Determining a calculation rule of a switching cycle meeting a condition of zero voltage turn-on and a mapping relationship between a capacitance value of the output capacitor and a voltage difference between the DC input voltage and the grid voltage according to the waveform data of the output capacitor voltage and the corresponding filter inductor current; A method, comprising: determining a duty ratio by a closed loop control mechanism; calculating a switching cycle according to a calculation rule of the switching cycle, a mapping relationship, a DC input voltage, a grid voltage, a grid-connected reference current and the duty ratio; and controlling a switching device to perform zero voltage turn-on according to the switching cycle and the duty ratio.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to the field of microgrids, and more particularly to a control method for grid-connected inverters and to grid-connected inverters. [Background technology]

[0002] Grid-connected inverters are widely used in the field of microgrid technology. A typical grid-connected inverter consists of four switching elements and a filter circuit. By controlling the high-frequency switching operation of the switching elements, it generates a high-frequency current, which is then used to form commercial frequency AC power that is supplied to the grid via the filter circuit. However, conventional hard-switching control methods result in large turn-on losses for the switching elements, which is detrimental to the development of higher frequency and higher efficiency grid-connected inverters. [Overview of the project]

[0003] The embodiment of the present invention aims to provide a control method for a grid-connected inverter and a grid-connected inverter in order to solve the problem of excessively large turn-on losses of the switching elements in conventional grid-connected inverters.

[0004] To solve the technical problems described above, the present invention provides the following technical solutions.

[0005] According to one aspect of the present invention, a DC voltage input source, switching elements S1 to S4, body diodes D1 to D4 and output capacitors C1 to C4 connected in parallel with the switching elements, and filter inductor L1 and filter capacitor C connected to a bridge leg on which switching elements S1 / S2 are installed. o2 Then, the filter inductor L2 and filter capacitor C are connected to the bridge leg on which the switching elements S3 / S4 are installed. o1 A control method for a grid-connected inverter comprising a grid, Based on the waveform data of the output capacitor voltage at both ends of the high-frequency operating switching element and the corresponding filter inductor current, the switching period T that satisfies the zero-voltage turn-on condition s Step A of determining the calculation rule of, and the mapping relationship between the capacitance value of the output capacitor, the DC input voltage, and the voltage difference between the system voltage and the system voltage, Step B of obtaining the DC input voltage, the system voltage, the system connection current, and the system connection reference current Step C of determining the high-frequency operating switching element according to the polarity of the system connection reference current Step D of determining the duty ratio by a closed-loop adjustment mechanism based on the system connection current and the system connection reference current The calculation rule of the switching period T s From the calculation rule of the switching period T, the mapping relationship, the DC input voltage, the system voltage, the system connection reference current, and the duty ratio, the switching period T s Step E of calculating The switching period T s Based on the switching period T and the duty ratio, step F of controlling the switching element to turn on with zero voltage is provided.

[0006] Optionally, step A includes Based on a set of waveform data in one switching period of the output capacitor voltage at both ends of the high-frequency operating switching element and the corresponding filter inductor current, determining the start / end time points of the switching period T that satisfies the zero-voltage turn-on condition s And Based on the start / end time points of the switching period T s Determining each period included in the switching period T s And the calculation rule for each period Based on the calculation rule for each period, obtaining the calculation rule of the switching period T s And including

[0007] Optionally, the switching period T that satisfies the zero-voltage turn-on conditions The start / end point is the point at which the filter inductor current and the output capacitor voltage in the set of waveform data simultaneously become zero.

[0008] Selectively, the switching period T s This is the ON period T of one switching element. on Then, there is a charging period T1 for one output capacitor and an off period T for one switching element. off This consists of one first resonance period T2, one body diode clamp period T3, and several second resonance periods T4.

[0009] Selectively, the switching period T s The calculation rules are: The file is JPEG0007897990000001.jpg9177, and here, JPEG0007897990000002.jpg33 has a switching period T s This is an adjustment value, the range of possible values ​​is an integer greater than or equal to zero, and L is the inductance value of the filter inductor. JPEG0007897990000003.jpg1384, i p V is the peak value of the filter inductor current, duty is the duty cycle, and V in V is a DC input voltage, grid is the system voltage, i ref V is the grid connection reference current, C is the capacitance value of the output capacitor, and the voltage difference V is also V. in -V grid and the mapping relationship C=f3(V in -V grid It is located in ).

[0010] Selectively, step A is, Based on multiple sets of waveform data at different switching periods, of the output capacitor voltage across a high-frequency operating switching element and the corresponding filter inductor current, different DC input voltages V in and a different system voltage V gridMeasure T4 under these conditions, and calculate the corresponding capacitance value C of the output capacitor from the measured T4, The aforementioned different DC input voltages V in and the aforementioned different system voltage V grid The difference with (V in -V grid ) is used as the input, and the different DC input voltages V in and the aforementioned different system voltage V grid The capacitance value C of the output capacitor calculated from the above is used as the output, and a polynomial fitting method is used to determine the capacitance value C of the output capacitor and the DC input voltage V. in and the system voltage V grid Mapping relationship with voltage difference C = f3(V in -V grid This includes seeking ) and further includes .

[0011] Selectively, step D is, The aforementioned grid connection current i grid and the aforementioned grid connection reference current i ref Based on this, the duty cycle is calculated using the following formula: JPEG0007897990000004.jpg539 Here, G P This is the transfer function of a closed-loop controller.

[0012] According to another aspect of the present invention, a DC voltage input source, switching elements S1 to S4, body diodes D1 to D4 and output capacitors C1 to C4 connected in parallel with the switching elements, and filter inductors L1 and filter capacitors C connected to a bridge leg on which switching elements S1 / S2 are installed. o2 Then, the filter inductor L2 and filter capacitor C are connected to the bridge leg on which the switching elements S3 / S4 are installed. o1 The system comprises a grid and a controller, and the controller has a switching period T that satisfies the condition of zero voltage turn-on. sA grid-connected inverter in which the calculation rules and the mapping relationship between the capacitance value of the output capacitor and the voltage difference between the DC input voltage and the grid voltage are predetermined, wherein the controller acquires the DC input voltage, the grid voltage, the grid connection current and the grid connection reference current, determines a high-frequency operating switching element according to the polarity of the grid connection reference current, determines the duty cycle by a closed-loop adjustment mechanism based on the grid connection current and the grid connection reference current, and the switching period T s From the calculation rules, the mapping relationship, the DC input voltage, the system voltage, the system connection reference current, and the duty cycle, the switching period T s To determine the switching period T s A grid-connected inverter is provided, which is used to control the switching element to turn on at zero voltage based on the duty cycle.

[0013] Selectively, the switching period T s The calculation rules are: The image is JPEG0007897990000005.jpg9177, where n is the switching period T. s This is an adjustment value, the range of possible values ​​is an integer greater than or equal to zero, and L is the inductance value of the filter inductor. JPEG0007897990000006.jpg1384, i p V is the peak value of the filter inductor current, duty is the duty cycle, and V in V is a DC input voltage, grid is the system voltage, i ref V is the grid connection reference current, C is the capacitance value of the output capacitor, and the voltage difference V is also V. in -V grid and the mapping relationship C=f3(V in -V grid It is located in ).

[0014] Selectively, the mapping relationship is Based on multiple sets of waveform data at different switching periods, of the output capacitor voltage across a high-frequency operating switching element and the corresponding filter inductor current, different DC input voltages V in and a different system voltage V grid Measure T4 under these conditions, and calculate the corresponding capacitance value C of the output capacitor from the measured T4, The aforementioned different DC input voltages V in and the aforementioned different system voltage V grid The difference with (V in -V grid ) is used as the input, and the different DC input voltages V in and the aforementioned different system voltage V grid The capacitance value C of the output capacitor calculated from the above is used as the output, and a polynomial fitting method is used to determine the capacitance value C of the output capacitor and the DC input voltage V. in and the system voltage V grid Mapping relationship with voltage difference C = f3(V in -V grid It is determined by a method that includes finding ).

[0015] The beneficial effects of the embodiments of the present invention are as follows. Unlike the prior art, in the embodiments of the present invention, first, based on the waveform data of the output capacitor voltage across the high-frequency operating switching element and the corresponding filter inductor current, the switching period T that satisfies the zero-voltage turn-on condition is determined. s The calculation rules and the mapping relationship between the capacitance value of the output capacitor and the voltage difference between the DC input voltage and the grid voltage are determined. Next, the DC input voltage, grid voltage, grid connection current, and grid connection reference current are obtained. Depending on the polarity of the grid connection reference current, a high-frequency operating switching element is determined. Based on the grid connection current and grid connection reference current, the duty cycle is determined by the closed-loop adjustment mechanism. Finally, the switching period T is determined. s From the calculation rules, mapping relationships, DC input voltage, grid voltage, grid connection reference current, and duty cycle, the switching period T is calculated. s Calculate the switching period T sA control method for a grid-connected inverter is provided, which controls a high-frequency operating switching element to turn on at zero voltage based on the duty cycle. The method of the present invention enables the switching element to turn on at zero voltage, effectively reducing losses due to the switching element's turn-on. [Brief explanation of the drawing]

[0016] One or more embodiments are illustrated by the corresponding figures in the accompanying drawings, but these illustrative descriptions are not limiting to the embodiments, and in the accompanying drawings, elements with the same reference numerals represent similar elements, and the figures in the accompanying drawings are not scale-limited unless otherwise specified. [Figure 1] This is a flowchart of a control method for a grid-connected inverter according to an embodiment of the present invention. [Figure 2] This is a schematic diagram of the topology configuration of a grid-connected inverter according to an embodiment of the present invention. [Figure 3] Figure 2 is a schematic diagram of the PWM drive signals for each switching element of the grid-connected inverter. [Figure 4] Figure 2 shows schematic diagrams of the waveforms of the output capacitor voltages VC1 / VC4 and filter inductor current iL1 when the grid-connected inverter is driven within a positive half-cycle of the commercial frequency while the grid voltage Vgrid is within the commercial frequency. [Figure 5] This figure shows the simulation results of the grid connection current using the control method of the grid-connected inverter shown in Figure 1. [Figure 6] Figure 1 shows the effect of simulation control using the control method for grid-connected inverters. [Figure 7] This is a schematic diagram of the structure of a grid-connected inverter according to an embodiment of the present invention. [Modes for carrying out the invention]

[0017] To further clarify the purpose, technical solutions, and advantages of the embodiments of the present invention, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments. However, it is clear that the embodiments described herein represent only a portion of the embodiments of the present invention and do not encompass all embodiments. All other embodiments obtained based on the embodiments of the present invention, assuming no creative work by those skilled in the art, fall within the scope of the protection of the present invention.

[0018] Furthermore, the technical features relating to each embodiment of the present invention described below can be combined with each other, provided they do not conflict with each other.

[0019] The steps shown in the flowchart in the attached drawings can be executed by a computer system, such as a series of computer executable instructions. Although the flowchart shows a logical order, it may be possible to execute the steps shown or described in an order different from the order specified herein. Example 1

[0020] Figure 1 is a flowchart of a control method for a grid-connected inverter according to an embodiment of the present invention. Referring to Figure 1, this method specifically includes the following steps.

[0021] Step S101: Based on the waveform data of the output capacitor voltage across the high-frequency operating switching element and the corresponding filter inductor current, the switching period T satisfies the zero-voltage turn-on condition. s The calculation rules and the mapping relationship between the capacitance value of the output capacitor and the voltage difference between the DC input voltage and the system voltage are determined.

[0022] In an embodiment of the present invention, the network topology configuration of the grid-connected inverter is shown in Figure 2. This grid-connected inverter includes a DC voltage input source, switching elements S1 to S4, body diodes D1 to D4 and output capacitors C1 to C4 connected in parallel to the switching elements S1 to S4, and filter inductors L1 and filter capacitors C connected to the bridge leg on which the switching elements S1 / S2 are installed. o2 Then, the filter inductor L2 and filter capacitor C are connected to the bridge leg on which the switching elements S3 / S4 are installed. o1 The system includes a filter inductor and a grid. To maintain topological symmetry, filter inductors L1 and L2 are selected to be of the same type, and output capacitors C1 to C4 are selected to be of the same type, i.e., L1=L2=L, C1=C2=C3=C4=C. Here, L is the inductance value of the filter inductor, and C is the capacitance value of the output capacitor.

[0023] Figure 3 is a schematic diagram of the PWM drive signals of each switching element of the grid-connected inverter in Figure 2, and as shown in Figure 3, here the grid voltage V grid and grid connection current i grid The phase angles are the same. System voltage V grid During the positive half-period of the commercial frequency, the grid connection reference current is positive, the pulse width of the high-frequency PWM (pulse width modulation) drive signals of switching elements S1 / S4 changes according to the sine law, S1 / S4 are driven in sync, and switching elements S2 / S3 remain off. In this case, current flows from the positive terminal of the DC voltage input source to the positive terminal of the grid via switching element S1 and filter inductor L1, and then returns from the negative terminal of the grid to the negative terminal of the DC voltage input source via filter inductor L2 and switching element S4. Grid voltage V gridDuring the negative half-period of the commercial frequency, the grid connection reference current is negative, the pulse width of the high-frequency PWM drive signal of switching elements S2 / S3 changes according to the sine law, S2 / S3 are driven in sync, and switching elements S1 / S4 remain off. In this case, current flows from the positive terminal of the DC voltage input source to the negative terminal of the grid via switching element S3 and filter inductor L2, and then returns from the positive terminal of the grid to the negative terminal of the DC voltage input source via filter inductor L1 and switching element S2.

[0024] In an embodiment of the present invention, the waveform data of the output capacitor voltage across the switching element operating at high frequency and the corresponding filter inductor current are defined as the output capacitor voltage V of the output capacitors C1 / C4 when the switching element S1 / S4 is operating at high frequency. c1 / V c4 and the filter inductor current i of filter inductor L1 L1 The first waveform data, or the output capacitor voltage V of output capacitors C2 / C3 when switching elements S2 / S3 are operating at high frequency. c2 / V c3 and the filter inductor current i of filter inductor L2 L2 This is the second waveform data. Due to the symmetry of the grid-connected inverter, the waveform configuration of the first and second waveform data are the same. Switching period T satisfies the zero-voltage turn-on condition. s When determining the calculation rules, it is sufficient to select and analyze any one set of waveform data within one switching period.

[0025] Specifically, first, based on a selected set of waveform data, the switching period T that satisfies the zero-voltage turn-on condition is determined. s Determine the start / end point of the switching period T, and then determine the switching period T. s Based on the start / end times, the switching period T s Determine each period included and the calculation rule for each period, and finally, determine the switching period T based on the calculation rule for each period. sFind the calculation rules. In Figure 2, one body diode and an output capacitor are connected in parallel to each switching element of the line-connected inverter. The voltage of this output capacitor is the same as the drain-source voltage across the corresponding switching element. That is, when the output capacitor voltage is zero, the drain-source voltage across the corresponding switching element also becomes zero. Therefore, the time point when the filter inductor current and the output capacitor voltage in the waveform data simultaneously become zero is the switching period T that satisfies the zero-voltage turn-on condition. s This is the start / end point of

[0026] <00​​​​​​​​​​​​​​​​​​​​​​​​​​​​C4 = 0

[0028] (2) Output capacitor charging period T1 The output capacitor charging period T1 starts at time t1 and ends at time t2. Specifically, at time t1, i L1 Then, the output capacitors C1 / C4 of S1 / S4 are charged, and V C1 / V C4 As it rises from zero, at time t2, V C1 =V C4 =V in This will result in i L1 is roughly i p It remains unchanged.

[0029] (3) Switching element off period T off Switching element off period T off It starts at time t2 and ends at time t3. Specifically, at time t2, i L1 ga i p It decreases linearly from time t3, and at time t3, i L1 It decreases to zero over a period of T. off Within, V C1 / V C4 ga V in It remains unchanged.

[0030] (4) First resonance period T2 The first resonant period T2 begins at time t3 and ends at time t4. Specifically, at time t3, the inverter enters the first resonant period, and L1, C1 and C4 form a resonant network, i L1 The current reverses direction and discharges through C1 / C4. At time t4, the voltage across C1 / C4 is completely discharged and returns to zero, i.e., V C1 =V C4 = 0

[0031] (5) Body diode clamp period T3 The body diode clamp period T3 starts at time t4 and ends at time t5. Specifically, at time t4, i L1The body diodes D1 / D4 are freewheeling, and D1 / D4 are V C1 / V C4 Clamp it to zero, i.e., V C1 =V C4 = 0, and at time t5, i L1 It rises linearly to zero.

[0032] (6) Second resonance period T4 The second resonance period T4 begins at time t5 and ends at time t6. Specifically, at time t5, the inverter enters the second resonance period, i L1 and V C1 / V C4 The amplitude changes sinusoidally according to the resonance period, and at time t6, i L1 and V C1 / V C4 Both resonate at zero, and at this time the switching element can be turned on at zero voltage. In this case, without external intervention, i L1 and V C1 / V C4 It continues to cycle through the second resonance period.

[0033] As can be seen from each period included in one switching cycle of the grid-connected inverter described above, the output capacitor voltage V C1 / V C4 and filter inductor current i L1 The time when both become zero simultaneously corresponds to the time t5 + nT4, where n is the switching period T. s This is an adjustment value, and its possible values ​​are non-negative integers. At any of these times, S1 / S4 can be turned on with zero voltage. Therefore, the switching period that satisfies the condition for zero voltage turn-on is T s =T on +T off It is +T1+T2+T3+nT4. Clearly, the switching period T s is the minimum T on +T off The formula is +T1+T2+T3. If you want to widen the switching period, you can set an appropriate n value according to the actual demand.

[0034] As can be seen from Figure 4, the switching period T s V C1 and V C4 The two remain in an equal state. The capacitance values ​​C1 / C4 of the output capacitors of the switching element are equal to the voltage across the capacitor V C1 / V C4 The capacitance value C of the output capacitor corresponding to the ON state of the switching element changes with the voltage V of that output capacitor. C It is a function of, that is, C = f1(V c )T4. Therefore, the second resonance period T4 is, JPEG0007897990000007.jpg939(1) It can be expressed as, and in the second resonance period T4, V C1 and V C4 and voltage difference V in -V grid There is a positive correlation between these two, JPEG0007897990000008.jpg647(2) It can be expressed as, and equation (1) further JPEG0007897990000009.jpg953(3) It can also be expressed as, Here, C is the voltage difference V in -V grid It is a function of . If T4 is known, C is uniquely determined. Thus, the output capacitance C and the DC input voltage V in and the system voltage V grid Mapping relationship with voltage difference C = f3(V in -V grid ) can be determined as follows:

[0035] First, the output capacitor voltage (e.g., V) across the high-frequency operating switching element. C1 / V C4 Based on multiple sets of waveform data of the ) and the corresponding filter inductor current at different switching periods, different DC input voltages V in and a different system voltage V gridUnder these conditions, measure T4, and from the measured T4, use equation (3) to calculate the corresponding capacitance value C of the output capacitor. Next, different DC input voltages V in and a different system voltage V grid The difference with (V in -V grid ) takes as input and a different DC input voltage V in and a different system voltage V grid The capacitance value C of the output capacitor calculated from the above is used as the output, and a polynomial fitting method is used to determine the capacitance value C of the output capacitor and the DC input voltage V. in and the system voltage V grid Mapping relationship with voltage difference C = f3(V in -V grid This mapping relationship is suitable not only for switching devices that differ from manufacturer to manufacturer, but also takes into account situations where the capacitance value of the output capacitor of the same switching element changes dynamically due to changes in the voltage across the capacitor.

[0036] Furthermore, based on this mapping relationship, according to Kirchhoff's voltage-current law, the switching element on-period T on Output capacitor charging period T1, switching element off period T off Therefore, the calculation rules for the first resonance period T2 and the body diode clamp period T3 can be determined, and specifically they are as follows.

[0037] JPEG0007897990000010.jpg1130(4) JPEG0007897990000011.jpg1130(5) JPEG0007897990000012.jpg1961(6) JPEG0007897990000013.jpg1259(7) JPEG0007897990000014.jpg1871(8) Here, i p This is the peak value of the filter inductor current. Switching period T s Inside, i pThe following relationship exists between the function and the duty cycle. JPEG0007897990000015.jpg1384(9)

[0038] This gives the transmission / reception period within the positive half-period of the commercial frequency. Each period contained in JPEG0007897990000016.jpg44 (i.e., T on , T off Calculation rules and mapping relationship C=f3(V) for T1, T2, T3, T4 in -V grid ) was determined. It can be seen that within the negative half-period of the commercial frequency, the calculation rule for the switching period is the same as the calculation rule within the positive half-period of the commercial frequency. The difference is that within the negative half-period of the commercial frequency, the system voltage V grid and grid connection reference current i ref The fact that it is a negative value. Therefore, in order to calculate it uniformly, the switching period T s According to the calculation rules, the system voltage V grid and grid connection reference current i ref Both take their absolute values.

[0039] Step S102: Obtain the DC input voltage, grid voltage, grid connection current, and grid connection reference current.

[0040] In an embodiment of the present invention, the grid-connected inverter further comprises a DC voltage sampling unit, a grid voltage sampling unit, and a grid-connected current sampling unit. Here, the DC voltage sampling unit is provided on both sides of the DC voltage input source and is used to collect the DC input voltage in real time. The grid voltage sampling unit is provided on both sides of the grid voltage and is used to collect the grid voltage in real time. The grid-connected current sampling unit is provided on the current output side of the inverter and is used to collect the grid-connected current in real time. In each switching cycle, the controller of the grid-connected inverter acquires the DC input voltage, grid voltage, and grid-connected current in real time using the DC voltage sampling unit, the grid voltage sampling unit, and the grid-connected current sampling unit.

[0041] The grid connection reference current is a set value, and its waveform changes sinusoidally within one cycle of the commercial frequency. The formula for calculating the grid connection reference current is: JPEG0007897990000017.jpg538(10) And here, I ref ωt is the amplitude of the grid connection reference current, and ωt is the phase of the grid connection reference current.

[0042] Step S103: Determine the switching element that operates at a high frequency according to the polarity of the grid connection reference current.

[0043] As shown in Figure 2, the grid-connected inverter controls the switching elements S1 / S4 to operate at a high frequency when the grid-connected reference current is positive, and controls the switching elements S2 / S3 to operate at a high frequency when the grid-connected reference current is negative.

[0044] Step S104: The duty cycle is determined by the closed-loop adjustment mechanism based on the grid connection current and the grid connection reference current.

[0045] In embodiments of the present invention, the closed-loop controller for grid-connected current is a PI (Proportion Integration) controller, a PID (Proportion Integration Differentiation) controller, etc. The formula for calculating the duty cycle is: JPEG0007897990000018.jpg539(11) And here, G P This is the transfer function of a closed-loop controller.

[0046] Step S105: The switching period T s From the calculation rules, the mapping relationship, the DC input voltage, the system voltage, the system connection reference current, and the duty cycle, the switching period T s To decide.

[0047] In the embodiments of the present invention, the switching period T satisfies the zero-voltage turn-on condition. s After determining the calculation rules and the mapping relationship between the output capacitor capacitance value and the voltage difference between the DC input voltage and the grid voltage, these are solidified in the grid-connected inverter controller. In each switching cycle, the controller acquires the necessary parameter information for the calculation rules and incorporates this parameter information into the switching cycle calculation rules to calculate the time for each switching cycle. The switching cycle T obtained as described above s According to the calculation rules, parameter information includes DC input voltage, grid voltage, grid connection reference current, and duty cycle.

[0048] Step S106: The switching period T s Based on the duty cycle, the switching element is controlled to turn on at zero voltage.

[0049] To make it easier to understand, the switching period that satisfies the zero-voltage turn-on condition. Once the calculation rules for JPEG0007897990000019.jpg44, and the mapping relationship between the capacitance value of the output capacitor and the voltage difference between the DC input voltage and the system voltage are established in the controller, the controller only needs to execute steps S102 to S106 in each switching cycle.

[0050] Referring to Figures 5 and 6, Figure 5 shows the simulation results of the grid connection current using the control method of the grid-connected inverter in Figure 1, and Figure 6 shows the effect of the simulation control using the control method of the grid-connected inverter in Figure 1. As can be seen from Figures 5 and 6, the control method of the grid-connected inverter of the present invention ensures synchronization between the grid connection current and the grid connection reference current while realizing that the switching elements turn on at zero voltage.

[0051] The control method for a grid-connected inverter according to an embodiment of the present invention first determines the switching period T that satisfies the zero-voltage turn-on condition based on the waveform data of the output capacitor voltage across the high-frequency operating switching element and the corresponding filter inductor current. s The calculation rules and the mapping relationship between the capacitance value of the output capacitor and the voltage difference between the DC input voltage and the grid voltage are determined. Next, the DC input voltage, grid voltage, grid connection current, and grid connection reference current are obtained. Depending on the polarity of the grid connection reference current, a high-frequency operating switching element is determined. Based on the grid connection current and grid connection reference current, the duty cycle is determined by the closed-loop adjustment mechanism. Finally, the switching period T is determined. s From the calculation rules, mapping relationships, DC input voltage, grid voltage, grid connection reference current, and duty cycle, the switching period T is calculated. s Calculate the switching period T s Based on the duty cycle, the high-frequency operating switching element is controlled to turn on at zero voltage. The method of the present invention enables the switching element to turn on at zero voltage, effectively reducing losses due to the switching element's turn-on. Example 2

[0052] According to an embodiment of the present invention, a grid-connected inverter is provided. Figure 7 is a schematic diagram of the structure of a grid-connected inverter according to an embodiment of the present invention. As shown in Figure 7, this grid-connected inverter comprises a DC voltage input source, switching elements S1 to S4, body diodes D1 to D4 and output capacitors C1 to C4 connected in parallel to the switching elements, and filter inductors L1 and filter capacitors C connected to a bridge leg on which switching elements S1 / S2 are installed. o2 Then, the filter inductor L2 and filter capacitor C are connected to the bridge leg on which the switching elements S3 / S4 are installed. o1 The system includes a grid and a controller, the controller having a switching period T that satisfies the condition of zero voltage turn-on. s The calculation rules, the capacitance value of the output capacitor, and the mapping relationship between the DC input voltage and the voltage difference between the DC input voltage and the system voltage are pre-set.

[0053] Specifically, the controller acquires the DC input voltage, grid voltage, grid connection current and grid connection reference current, determines a high-frequency operating switching element according to the polarity of the grid connection reference current, determines the duty cycle by a closed-loop adjustment mechanism based on the grid connection current and the grid connection reference current, and controls the switching period T s From the calculation rules, the mapping relationship, the DC input voltage, the system voltage, the system connection reference current, and the duty cycle, the switching period T s To determine the switching period T s This is used to control the switching element so that it turns on at zero voltage, based on the duty cycle.

[0054] In embodiments of the present invention, the grid-connected inverter further comprises a DC voltage sampling unit, a grid voltage sampling unit, and a grid-connected current sampling unit. Here, the DC voltage sampling unit is installed on both sides of the DC voltage input source and is used to collect the DC input voltage in real time. The grid voltage sampling unit is installed on both sides of the grid voltage and is used to collect the grid voltage in real time. The grid-connected current sampling unit is installed on the current output side of the inverter and is used to collect the grid-connected current in real time. In each switching cycle, the controller of the grid-connected inverter acquires the DC input voltage, grid voltage, and grid-connected current in real time using the DC voltage sampling unit, the grid voltage sampling unit, and the grid-connected current sampling unit.

[0055] In embodiments of the present invention, based on a set of waveform data for one switching period of the output capacitor voltage across a high-frequency operating switching element and the corresponding filter inductor current, a switching period T that satisfies the zero-voltage turn-on condition is determined. s Determine the start / end time of the switching period T, and based on this time, s Determine each period included and the calculation rule for each period, and determine the switching period T based on the calculation rule for each period.s We will determine the calculation rule for the switching period T. s This is the ON period T of one switching element. on Then, there is a charging period T1 for one output capacitor and an off period T for one switching element. off It consists of one first resonance period T2, one body diode clamp period T3, and several second resonance periods T4, that is, T s =T on +T off +T1+T2+T3+nT4, where n is a non-negative integer.

[0056] In the embodiments of the present invention, the switching period T s The calculation rules are: The file is JPEG0007897990000020.jpg9177, and here, JPEG0007897990000021.jpg33 has a switching period T s This is an adjustment value, the range of possible values ​​is an integer greater than or equal to zero, and L is the inductance value of the filter inductor. The filename is JPEG0007897990000022.jpg1384. i p V is the peak value of the filter inductor current, duty is the duty cycle, and V in V is a DC input voltage, grid is the system voltage, i ref V is the grid connection reference current, C is the capacitance value of the output capacitor, and the voltage difference V is also V. in -V grid and the mapping relationship C=f3(V in -V grid It is located in ).

[0057] As can be seen from the calculation formula above, if T4 is known, C is uniquely determined. Therefore, the output capacitance C and the DC input voltage V in and the system voltage V grid Mapping relationship with voltage difference C = f3(V in -V grid ) can be determined as follows:

[0058] First, the output capacitor voltage (e.g., V) across the high-frequency operating switching element. C1 / V C4 Based on multiple sets of waveform data of the ) and the corresponding filter inductor current at different switching periods, different DC input voltages V in and a different system voltage V grid Under these conditions, measure T4, and from the measured T4, use equation (3) to calculate the corresponding capacitance value C of the output capacitor. Next, different DC input voltages V in and a different system voltage V grid The difference with (V in -V grid ) takes as input and a different DC input voltage V in and a different system voltage V grid The capacitance value C of the output capacitor calculated from the above is used as the output, and a polynomial fitting method is used to determine the capacitance value C of the output capacitor and the DC input voltage V. in and the system voltage V grid Mapping relationship with voltage difference C = f3(V in -V grid )

[0059] Finally, it should be noted that the above embodiments are merely illustrative of the technical solutions of the present invention and do not limit them. In the concept of the present invention, it is possible to combine the technical features of the above embodiments or different embodiments, the steps may be carried out in any order, and there are many other variations in different aspects of the present invention as described above, but for the sake of brevity, their details are not described. The present invention has been described in detail with reference to the above embodiments, but those skilled in the art should understand that it is possible to modify the technical solutions described in each of the above embodiments or to replace some of their technical features with equivalents. These modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of each embodiment of the present application.

Claims

1. A control method for a grid-connected inverter, Based on the waveform data of the output capacitor voltage across a high-frequency operating switching element and the corresponding filter inductor current, a switching period T that satisfies the zero-voltage turn-on condition is determined. ｓ Step A involves determining the calculation rules and the mapping relationship between the capacitance value of the output capacitor and the voltage difference between the DC input voltage and the system voltage. Step B involves obtaining the DC input voltage, grid voltage, grid connection current, and grid connection reference current. Step C, which determines a switching element that operates at a high frequency according to the polarity of the grid connection reference current, Step D, in which the duty cycle is determined by a closed-loop adjustment mechanism based on the grid connection current and the grid connection reference current, The aforementioned switching period T ｓ From the calculation rules, the mapping relationship, the DC input voltage, the system voltage, the system connection reference current, and the duty cycle, the switching period T ｓ Step E for calculating, The aforementioned switching period T ｓ A control method for a grid-connected inverter, characterized by including step F, which controls the switching element to turn on at zero voltage based on the duty cycle.

2. Step A is, Based on a set of waveform data for one switching cycle of the output capacitor voltage across a high-frequency operating switching element and the corresponding filter inductor current, a switching period T that satisfies the zero-voltage turn-on condition is determined. ｓ To determine the start and end points, The aforementioned switching period T ｓ Based on the start and end times, the switching period T ｓ Determine each period included and the calculation rules for each of those periods, Based on the calculation rules for each period, the switching period T ｓ The method according to claim 1, characterized by including the calculation rule for the following.

3. The switching period T that satisfies the condition of zero-voltage turn-on ｓ The start point and the end point thereof are the points at which the filter inductor current and the output capacitor voltage in the set of waveform data simultaneously become zero, and the method according to claim 2, characterized in that.

4. The aforementioned switching period T ｓ This is the ON period T of one switching element. ｏｎ And, the charging period T of one output capacitor １ And, the off period T of one switching element ｏｆｆ And, one first resonance period T ２ And, one body diode clamp period T ３ And several second resonance periods T ４ The method according to 2 or 3, characterized by comprising the above.

5. The aforementioned switching period T ｓ The calculation rules are: Here, n is the switching period T. ｓ This is an adjustment value, the range of possible values ​​is an integer greater than or equal to zero, and L is the inductance value of the filter inductor. i ｐ V is the peak value of the filter inductor current, duty is the duty cycle, and V ｉｎ V is a DC input voltage, ｇｒｉｄ is the system voltage, i ｒｅｆ V is the grid connection reference current, C is the capacitance value of the output capacitor, and the voltage difference V is the voltage difference. ｉｎ -V ｇｒｉｄ and mapping relationship C = f ３ (V ｉｎ -V ｇｒｉｄ The method according to 4, characterized in that it is located in ).

6. Step A is, Based on multiple sets of waveform data at different switching periods, of the output capacitor voltage across a high-frequency operating switching element and the corresponding filter inductor current, different DC input voltages V ｉｎ and a different system voltage V ｇｒｉｄ Second resonance period T under the conditions ４ The second resonance period T was measured. ４ From that, calculate the capacitance value C of the corresponding output capacitor, The aforementioned different DC input voltages V ｉｎ and the aforementioned different system voltage V ｇｒｉｄ The difference with (V ｉｎ -V ｇｒｉｄ The input is the different DC input voltage V ｉｎ and the aforementioned different system voltage V ｇｒｉｄ The capacitance value C of the output capacitor calculated from the above is used as the output, and a polynomial fitting method is used to determine the capacitance value C of the output capacitor and the DC input voltage V. ｉｎ and the system voltage V ｇｒｉｄ Mapping relationship with voltage difference C = f ３ (V ｉｎ -V ｇｒｉｄ The method according to claim 5, further comprising the steps of obtaining the following:

7. Step D is, The aforementioned grid connection current i ｇｒｉｄ and the aforementioned grid connection reference current i ｒｅｆ Based on this, the duty cycle is calculated using the following formula: Here, G Ｐ The method according to claim 5, characterized in that is the transfer function of a closed-loop controller.

8. A controller is provided, and the controller has a switching period T that satisfies the condition of zero voltage turn-on. ｓ The calculation rules, the capacitance value of the output capacitor, and the mapping relationship between the DC input voltage and the voltage difference between the DC input voltage and the system voltage are predetermined. The controller acquires the DC input voltage, grid voltage, grid connection current and grid connection reference current, determines a high-frequency operating switching element according to the polarity of the grid connection reference current, determines the duty cycle by a closed-loop adjustment mechanism based on the grid connection current and the grid connection reference current, and the switching period T ｓ From the calculation rules, the mapping relationship, the DC input voltage, the system voltage, the system connection reference current, and the duty cycle, the switching period T ｓ Calculating the switching period T ｓ A grid-connected inverter characterized by being used to control the switching element to turn on at zero voltage based on the duty cycle.

9. The switching period The calculation rules are: Here, n is the switching period T. ｓ This is an adjustment value, the range of possible values ​​is an integer greater than or equal to zero, and L is the inductance value of the filter inductor. i ｐ V is the peak value of the filter inductor current, duty is the duty cycle, and V ｉｎ V is a DC input voltage, ｇｒｉｄ is the system voltage, i ｒｅｆ V is the grid connection reference current, C is the capacitance value of the output capacitor, and the voltage difference V is the voltage difference. ｉｎ -V ｇｒｉｄ and mapping relationship C = f ３ (V ｉｎ -V ｇｒｉｄ A grid-connected inverter as described in claim 8, as described in the following feature.

10. The aforementioned mapping relationship is, Based on multiple sets of waveform data at different switching periods, of the output capacitor voltage across a high-frequency operating switching element and the corresponding filter inductor current, different DC input voltages V ｉｎ and a different system voltage V ｇｒｉｄ Second resonance period T under the conditions ４ The second resonance period T was measured. ４ From that, calculate the capacitance value C of the corresponding output capacitor, The aforementioned different DC input voltages V ｉｎ and the aforementioned different system voltage V ｇｒｉｄ The difference with (V ｉｎ -V ｇｒｉｄ The input is the different DC input voltage V ｉｎ and the aforementioned different system voltage V ｇｒｉｄ The capacitance value C of the output capacitor calculated from the above is used as the output, and a polynomial fitting method is used to determine the capacitance value C of the output capacitor and the DC input voltage V. ｉｎ and the system voltage V ｇｒｉｄ Mapping relationship with voltage difference C = f ３ (V ｉｎ -V ｇｒｉｄ The grid-connected inverter according to claim 9, characterized in that it is determined by a method including determining ).

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