Grid-connected inverter control method and grid-connected inverter

Abstract

The application discloses a grid-connected inverter control method and a grid-connected inverter, and the method comprises the following steps: determining a first adjustment coefficient of a grid-connected reference current based on a direct-current input voltage and a grid voltage; determining a grid-connected original reference current according to the grid voltage and preset grid-connected power; obtaining a grid-connected final reference current based on the first adjustment coefficient and the grid-connected original reference current; determining a high-frequency action switch tube, a switching period and a duty cycle of the switch tube based on the direct-current input voltage, the grid voltage and the grid-connected final reference current; and controlling the switch tube to be turned on and turned off based on the switching period and the duty cycle of the switch tube. The method can simultaneously adapt to the adjustment of the output current when the grid voltage is in a steady state or a surge state by calculating a specific adjustment coefficient; and when the grid voltage is in the surge state, the output current size can be inhibited to avoid the output current from being greatly distorted, and the inverter is off-grid or the device is damaged.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of micro-grid, in particular to a grid-connected inverter control method and a grid-connected inverter. BACKGROUND

[0002] In modern power electronic systems, single-phase grid-connected inverters are an important component, especially in distributed generation, solar photovoltaic systems and UPS (Uninterruptible Power Supply) applications, which are mainly used to convert DC power into AC power. In order to improve energy conversion efficiency and meet the quality requirements of grid-connected current waveform, the control strategy of grid-connected inverter is particularly important. Among many control strategies, DCM (Discontinuous conduction mode) control strategy is a method based on direct control of inverter output current, which adjusts the switching state of the inverter to control the output current to achieve accurate tracking of the current.

[0003] The inverter control strategy based on DCM theory calculation can improve the conversion efficiency under certain working conditions, but due to its open-loop control characteristics, it lacks real-time feedback mechanism and cannot detect and correct output deviation, resulting in poor adaptability when the grid voltage suddenly changes, which may cause output current distortion to worsen, and even cause the inverter to be off-grid or device damage in severe cases. SUMMARY

[0004] Embodiments of the present application aim to provide a grid-connected inverter control method and a grid-connected inverter to solve the problem that the grid-connected inverter based on DCM control in the prior art cannot effectively adapt to the sudden change of grid voltage, thereby causing the output current to distort and worsen.

[0005] To solve the above technical problems, the technical solutions of the embodiments of the present application are as follows:

[0006] According to an aspect of the present application, a grid-connected inverter control method is provided, the method comprising:

[0007] obtaining a DC input voltage and a grid voltage, and determining a first adjustment coefficient of a grid-connected reference current based on the DC input voltage and the grid voltage;

[0008] determining a grid-connected original reference current according to the grid voltage and a preset grid-connected power;

[0009] obtaining a grid-connected final reference current based on the first adjustment coefficient and the grid-connected original reference current;

[0010] The high-frequency action switch tube and the switching period and duty cycle of the switch tube are determined based on the DC input voltage, the grid voltage and the grid-connected final reference current, and the switch tube is controlled to turn on and turn off based on the switching period and duty cycle of the switch tube.

[0011] Optionally, the calculation formula of the first adjustment coefficient is:

[0012]

[0013] wherein, the first adjustment coefficient is, the DC input voltage is, the grid voltage is, the average value of the DC input voltage is, the effective value of the grid voltage is.

[0014] Optionally, the grid-connected inverter comprises an LC low-pass filter, and the grid-connected final reference current is obtained based on the first adjustment coefficient and the grid-connected original reference current, comprising:

[0015] The original switching frequency of the switch tube is determined according to the DC input voltage, the grid voltage and the grid-connected original reference current;

[0016] The second adjustment coefficient of the grid-connected reference current is determined according to the original switching frequency of the switch tube and the cutoff frequency of the LC low-pass filter;

[0017] The grid-connected final reference current is obtained based on the first adjustment coefficient, the second adjustment coefficient and the grid-connected original reference current.

[0018] Optionally, the cutoff frequency of the LC low-pass filter is calculated based on the inductance value and the capacitance value of the LC low-pass filter.

[0019] Optionally, the calculation formula of the second adjustment coefficient is:

[0020]

[0021] wherein, the second adjustment coefficient is, the original switching frequency of the switch tube is, the cutoff frequency of the LC low-pass filter is.

[0022] Optionally, the grid-connected final reference current is obtained based on the first adjustment coefficient, the second adjustment coefficient and the grid-connected original reference current, comprising:

[0023] The comprehensive adjustment coefficient is obtained based on the first adjustment coefficient and the second adjustment coefficient.

[0024] The grid-connected final reference current is obtained based on the comprehensive adjustment coefficient and the grid-connected original reference current.

[0025] Optionally, the original switching frequency of the switching tube is determined according to the DC input voltage, the grid voltage and the grid-connected original reference current, and the method comprises the following steps:

[0026] The working mode of the grid-connected inverter is determined based on the polarity of the DC input voltage and the grid-connected original reference current, and the working mode comprises an inverting mode and a rectifying mode.

[0027] The original switching frequency of the switching tube is determined based on the period calculation rule corresponding to the zero-voltage turn-on condition of the DC input voltage, the grid voltage and the working mode.

[0028] Optionally, the grid-connected inverter further comprises a DC voltage input source, a switching tube and a body diode connected in parallel with the switching tube and an output capacitor , a filter inductor and a filter capacitor connected to the bridge arm in which the switching tube and are located, a filter inductor and a filter capacitor connected to the bridge arm in which the switching tube and are located, and a grid , and the calculation rule of the first switching period of the grid-connected inverter satisfying the zero-voltage turn-on condition in the inverting mode is as follows:

[0029]

[0030] The calculation rule of the second switching period of the grid-connected inverter satisfying the valley voltage turn-on condition in the rectifying mode is as follows:

[0031]

[0032] wherein, , , is the peak value of the filter inductor current in the inverting mode, is the peak value of the filter inductor current in the rectifying mode, is the DC input voltage, is the grid voltage, is the inductance value of the filter inductor and , for output capacitor and capacitance value of the capacitor, for grid-connected reference current.

[0033] According to another aspect of the present application, a grid-connected inverter is provided, comprising a controller,

[0034] the controller is configured to acquire a DC input voltage and a grid voltage, determine a first adjustment coefficient of a grid-connected reference current based on the DC input voltage and the grid voltage, determine a grid-connected original reference current based on the grid voltage and a preset grid-connected power, obtain a grid-connected final reference current based on the first adjustment coefficient and the grid-connected original reference current, determine a high-frequency action switch tube, a switching period and a duty cycle of the switch tube based on the DC input voltage, the grid voltage and the grid-connected final reference current, and control the switch tube to turn on and turn off based on the switching period and the duty cycle of the switch tube.

[0035] Optionally, the grid-connected inverter further comprises an LC low-pass filter, and the controller is further configured to determine an original switching frequency of the switch tube based on the DC input voltage, the grid voltage and the grid-connected original reference current, determine a second adjustment coefficient of the grid-connected reference current based on the original switching frequency of the switch tube and a cutoff frequency of the LC low-pass filter, and obtain the grid-connected final reference current based on the first adjustment coefficient, the second adjustment coefficient and the grid-connected original reference current.

[0036] The present application has the following advantages. Unlike the prior art, the present application provides a grid-connected inverter control method. First, a DC input voltage and a grid voltage are acquired, and a first adjustment coefficient of a grid-connected reference current is determined based on the DC input voltage and the grid voltage. Then, a grid-connected original reference current is determined based on the grid voltage and a preset grid-connected power, and a grid-connected final reference current is obtained based on the first adjustment coefficient and the grid-connected original reference current. Finally, a high-frequency action switch tube, a switching period and a duty cycle of the switch tube are determined based on the DC input voltage, the grid voltage and the grid-connected final reference current, and the switch tube is controlled to turn on and turn off based on the switching period and the duty cycle of the switch tube. The method of the present application does not need to detect abnormal grid conditions. By calculating a specific adjustment coefficient, the output current can be adjusted when the grid voltage is in a steady state or a surge state. When the grid voltage is in a surge state, the output current can be suppressed to avoid large distortion of the output current, which can cause the inverter to be disconnected from the grid or cause damage to the device. BRIEF DESCRIPTION OF DRAWINGS

[0037] One or more embodiments are illustrated by way of example in the figures that form a part of this patent specification. These examples are not intended to limit the scope of embodiments to their details. Like numbers refer to like elements in the drawings where appropriate. The drawings provided are for purposes of illustration only and in no way limit the claimed application.

[0038] Figure 1 is a topological structure diagram of a grid-connected inverter provided by embodiments of the application;

[0039] Figure 2 is a structure diagram of a controller provided by embodiments of the application;

[0040] Figure 3 is a flow chart of a grid-connected inverter control method provided by embodiments of the application;

[0041] Figure 4 is Figure 1 a PWM drive signal diagram of a switch tube of a grid-connected inverter in region A;

[0042] Figure 5 is Figure 4 a first group of waveform diagrams of region A switch tube drive, drain-source voltage and first filter inductor current in region A;

[0043] Figure 6 is Figure 4 a second group of waveform diagrams of region A switch tube drive, drain-source voltage and first filter inductor current in region A;

[0044] Figure 7 is Figure 4 a first group of waveform diagrams of region B switch tube drive, drain-source voltage and second filter inductor current in region B;

[0045] Figure 8 is Figure 4 a second group of waveform diagrams of region B switch tube drive, drain-source voltage and second filter inductor current in region B;

[0046] Figure 9 is Figure 4 a group of waveform diagrams of region C switch tube drive, drain-source voltage and second filter inductor current in region C;

[0047] Figure 10 is Figure 4 the middle region D switch tube drive, drain-source voltage and the first filter inductance current a set of waveform schematic diagram;

[0048] Figure 11 is the refinement process provided by the embodiment of the present application for step S303;

[0049] Figure 12 is the relationship diagram between the switching frequency and the grid voltage provided by the embodiment of the present application;

[0050] Figure 13 is the relationship diagram between the duty cycle and the grid voltage provided by the embodiment of the present application;

[0051] Figure 14a is the oscilloscope measurement diagram of the traditional control method at the moment of the grid voltage surge;

[0052] Figure 14b is the oscilloscope measurement diagram of the control method of the present application at the moment of the grid voltage surge. DETAILED DESCRIPTION

[0053] In order to make the purpose, technical scheme and advantages of the embodiments of the present application clearer, the technical scheme in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are some embodiments of the present application, but not all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0054] In addition, the technical features involved in each of the embodiments of the present application described below can be combined with each other as long as there is no conflict.

[0055] It should be noted that the steps shown in the flowchart of the drawings can be executed in a computer system such as a set of computer executable instructions, and although the logical order is shown in the flowchart, in some cases, the steps shown or described herein can be executed in an order different from that shown herein.

[0056] Please refer to Figure 1 , Figure 1 is the topological structure schematic diagram of the grid-connected inverter provided by the embodiment of the present application. As Figure 1 shown, the grid-connected inverter includes an inverter circuit 10, a controller 20, a direct current voltage sampling unit 30, a grid voltage sampling unit 40 and an LC low-pass filter 50. Among them, the inverter circuit 10 includes a direct current voltage input source, a switch tube 、 、 、 switching tube (hereinafter referred to as switching tube ) and body diode (hereinafter referred to as body diode ) connected in parallel with switching tube 、 、 、 body diode (hereinafter referred to as body diode ) and output capacitor (hereinafter referred to as output capacitor 、 、 、 output capacitor (hereinafter referred to as output capacitor ) connected to switching tube and bridge arm, filter inductor and filter capacitor , filter inductor and connected to switching tube and bridge arm, filter inductor and . In order to maintain the symmetry of the topology, filter inductor and are selected to be consistent, and output capacitor is also selected to be consistent, that is: and . Among them, is the inductance value of filter inductor is the capacitance value of output capacitor and .

[0057] DC voltage sampling unit 30 is arranged on both sides of the DC voltage input source, which is used to collect the DC input voltage in real time; the grid voltage sampling unit 40 is arranged on both sides of the grid voltage, which is used to collect the grid voltage in real time.

[0058] The controller 20 is connected with the inverter circuit 10, the DC voltage sampling unit 30 and the grid voltage sampling unit 40 respectively. Specifically, the controller 20 is connected with the switching tube in the inverter circuit 10, and controls the conduction and turn-off of the switching tube based on the built-in control program. In some embodiments, the controller can adopt microcontroller unit (MCU) or digital signal processing (DSP) controller, etc.

[0059] LC low-pass filter 50 includes inductor , inductor and a capacitor where the inductance for suppressing common mode. LC low pass filter 50 is used to allow low frequency signals to pass while attenuating high frequency signals, with a cut-off frequency higher than the fundamental frequency (e.g. 50 Hz) and much less than the switching frequency (e.g. 10 kHz), typically 1-2 kHz. For the LC low pass filter, signals above the cut-off frequency are attenuated, and signals below the cut-off frequency can pass substantially without attenuation.

[0060] Please refer to Figure 2 , Figure 2 An exemplary structure of the controller 20 is shown. As shown in Figure 2 , the controller 20 includes at least one processor 21 and a memory 22, where the memory 22 can be built-in in the controller 20, or external to the controller 20, and the memory 22 can also be a remotely arranged memory connected to the controller 20 through a network.

[0061] The memory 22 is a non-volatile computer readable storage medium, which can be used to store non-volatile software programs, non-volatile computer executable programs and modules. The memory 22 can include a program storage area and a data storage area, where the program storage area can store an operating system and at least one application required by a function; the data storage area can store data created according to the use of the terminal, etc. In addition, the memory 22 can include a high-speed random access memory, and can also include a non-volatile memory, such as at least one magnetic disk storage device, a flash memory device, or other non-volatile solid-state memory device. In some embodiments, the memory 22 can optionally include a memory arranged remotely relative to the processor 21, and these remote memories can be connected to the terminal through a network. Examples of the above network include but are not limited to the Internet, an intranet, a local area network, a mobile communication network, and combinations thereof.

[0062] The processor 21 executes various functions of the terminal and processes data by running or executing software programs and / or modules stored in the memory 22, and calling data stored in the memory 22, thereby overall monitoring the terminal, such as implementing the grid-connected inverter control method according to any embodiment of the present application.

[0063] The processor 21 can be one or more, Figure 2The processor 21 and the memory 22 can be connected by a bus or other means. The processor 21 can include a central processing unit (CPU), a digital signal processor (DSP), an application-specific integrated circuit (ASIC), a controller, a field programmable gate array (FPGA) device, or the like. The processor 21 can also be implemented as a combination of computing devices, such as a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration.

[0064] Please refer to Figure 3 , Figure 3 is a grid-connected inverter control method provided by an embodiment of the application. The grid-connected inverter control method can be applied to a controller of a grid-connected inverter as shown in Figure 1 , and the method specifically includes the following steps.

[0065] In step S301, a DC input voltage and a grid voltage are acquired, and a first adjustment coefficient of a grid-connected reference current is determined based on the DC input voltage and the grid voltage.

[0066] Specifically, a DC voltage sampling unit of the grid-connected inverter transmits a real-time collected DC input voltage to the controller. A grid voltage sampling unit of the grid-connected inverter transmits a real-time collected grid voltage to the controller. The controller determines the first adjustment coefficient of the grid-connected reference current based on the DC input voltage and the grid voltage. In an embodiment, the calculation formula of the first adjustment coefficient is as follows:

[0067] (1)

[0068] wherein, is the first adjustment coefficient, is an instantaneous value of the DC input voltage (at the moment when the grid voltage suddenly changes), is an instantaneous value of the grid voltage (at the moment when the grid voltage suddenly changes), is an average value of the DC input voltage, is a valid value of the grid voltage. The average value of the DC input voltage is the real-time collected DC input voltage in one power frequency cycle is obtained by accumulation and averaging, and the valid value of the grid voltage is obtained by taking the root mean square of the real-time collected grid voltage in one power frequency cycle.

[0069] For a single-phase grid-connected inverter, when the grid voltage is in a sudden surge transient state, the power output demand instantaneously increases, exceeding the supply capacity of the DC side power supply, resulting in that the DC input voltage is pulled down, so that the DC input voltage is greater than the grid voltage The difference decreases. This is because transient changes affect the average value of the DC input voltage. and the effective value of grid voltage The impact is relatively small; therefore, the average DC input voltage... With the effective value of the grid voltage The difference remains approximately constant. In summary, the instantaneous value of the DC input voltage... Instantaneous value of grid voltage The difference between the average DC input voltage and the average DC input voltage With the effective value of the grid voltage The ratio of the differences decreases, that is... Decrease. This is understandable; the instantaneous value of the DC input voltage... Always greater than the instantaneous value of the grid voltage Average DC input voltage It is also always greater than the effective value of the grid voltage. ,Right now Therefore, As the first adjustment coefficient .

[0070] When the grid voltage is in a steady state, the coefficient (At this point, the grid-connected reference current is basically not adjusted.) When the grid voltage is in a surge transient state, the coefficient... And coefficient The coefficient varies with the magnitude of the voltage surge in the power grid; the larger the surge, the larger the coefficient. The smaller the value, the more adaptively the grid-connected reference current is reduced, suppressing the output current and preventing large distortions in the output current that could cause the inverter to disconnect from the grid or damage the components.

[0071] Step S302: Determine the original reference current for grid connection based on the grid voltage and the preset grid-connected power.

[0072] First, based on the user-defined grid-connected power and the sampled grid voltage, the amplitude of the original grid-connected reference current is calculated using the following formula:

[0073] (2)

[0074] in, The amplitude of the original reference current for grid connection. For grid-connected power, This is the effective value of the grid voltage.

[0075] Secondly, the initial reference current for grid-connected inverters differs depending on whether they operate at unity power factor or non-unity power factor, as detailed below:

[0076] If the grid-connected inverter operates at unity power factor, the initial grid-connected reference current can be expressed as:

[0077] (3)

[0078] If the grid-connected inverter operates at a non-unity power factor, the initial grid-connected reference current can be expressed as:

[0079] (4)

[0080] in, For grid connection, the original reference current, The phase of the grid-connected reference current, the phase difference between voltage and current ( The cosine of the power factor is denoted by the symbol . This indicates that it is usually preset by the user.

[0081] Step S303: Based on the first adjustment coefficient and the original grid-connected reference current, the final grid-connected reference current is obtained.

[0082] In one embodiment, the original grid-connected reference current is multiplied by a first adjustment coefficient to obtain the final grid-connected reference current, as follows:

[0083] (5)

[0084] in, This is the final reference current for grid connection.

[0085] Step S304: Based on the DC input voltage, grid voltage, and grid-connected final reference current, determine the high-frequency operating switch and its switching period and duty cycle. Based on the switching period and duty cycle of the switch, control the switch to turn on and off.

[0086] In this embodiment, a DCM-based control strategy is used to determine the switching period and duty cycle of the switching transistor. The specific method is as follows:

[0087] like Figure 4 As shown, based on the grid voltage and grid-connected final reference current The polarity of the frequency can divide a power frequency cycle into four regions: A, B, C, and D. In regions A and C, The grid-connected inverter is in inverter mode; in areas B and D, The grid-connected inverter operates in rectifier mode.

[0088] Please see Figure 5 , Figure 5 yes Figure 4 Middle Zone A Switch Drive, drain-source voltage and the current of the first filter inductor The first set of waveform diagrams. In region A, Therefore, the switching transistor High-frequency operation, switching transistor The circuit remains off, and the grid-connected current passes through a filter inductor before flowing into the grid. This generates the first filter inductor current. .like Figure 5 As shown, a switching cycle in region A can include the following five stages:

[0089] (1) Switching transistor conduction stage

[0090] Switch conduction stage from From the moment to The moment has ended. Specifically, in At any moment, the switching transistor / When the circuit is turned on, the current in the first filter inductor... It increases linearly from zero to... time, / Turn off, Increase to peak current Due to the switching transistor / It remains in the on state, therefore the drain-source voltage According to Kirchhoff's voltage and current laws, The calculation formula is:

[0091] (6)

[0092] In equation (6), , The current of the first filter inductor peak value DC input voltage This is the grid voltage. For filter inductors and inductance value, For output capacitor and The capacitance value.

[0093] (2) Switching transistor turn-off stage

[0094] Switching transistor turn-off stage from From the moment to The moment has ended. Specifically, in time, Give / Output capacitor Charge, rise from It begins to decrease linearly, until... time, Reduce to zero. Within a time period. Inside, Keep Unchanged. According to Kirchhoff's voltage and current laws, The calculation formula is:

[0095] (7)

[0096] It should be noted that during the turn-off phase of the switching transistor... Previously, it could also include an output capacitor. Charging phase .because The duration of each phase is almost negligible; therefore, to simplify calculations, the switching cycle is not included in the calculation in some embodiments. In other embodiments, to obtain a more accurate switching cycle, [the calculation is performed]. The duration of each phase is included in the calculation of the switching cycle. Specifically, for example... Figure 6 As shown, the output capacitor Charging phase from From the moment to The moment has ended. time, Give / Output capacitor Charge, Starting from zero, rising to... time, During this phase, Approximate preservation Unchanged. According to Kirchhoff's voltage and current laws, The calculation formula is:

[0097] (8)

[0098] (3) First resonance stage

[0099] First resonance phase from From the moment to The moment has ended. Specifically, in At that moment, the inverter enters the first resonance stage. , and Forming a resonant network, Reverse and give discharge. At time, the voltage across the body diode is completely discharged to zero, i.e. According to Kirchhoff's voltage-current law, the calculation formula is:

[0100] (9)

[0101] (4) Clamping phase of the body diode

[0102] Clamping phase of the body diode starts from the time ends at the time Specifically, at the time , the current continues to flow through the body diode , and is clamped to zero, i.e. , at the time , the voltage rises linearly to zero. According to Kirchhoff's voltage-current law, the calculation formula is:

[0103] (10)

[0104] (5) Second resonance phase

[0105] Second resonance phase starts from the time ends at the time Specifically, at the time , the inverter enters the second resonance phase, and the amplitudes change sinusoidally with the resonance period, and at the time , both and resonate to zero, at which time the / can be turned on with zero voltage. According to Kirchhoff's voltage-current law, the calculation formula is:

[0106] (11)

[0107] It can be understood that, in the inverter mode, without external intervention, and will always be in the second resonance phase.

[0108] According to the drain-source voltage Figure 5 in each phase in the inverter mode in ​​and the first filter inductor current The time when the drain-source voltage and the first filter inductor current are both zero is +n corresponding time. Wherein, n is the adjustment value of the switching period, and its value range is an integer greater than or equal to zero. At these times, zero voltage turn-on / can be realized. Therefore, in the inverter mode, the first switching period satisfying the zero voltage turn-on condition is . Obviously, the minimum switching period is If you want to increase the switching period, you can set the appropriate n value according to actual needs.

[0109] Figure 7 Please refer to Figure 7 , Figure 4 is the first group of waveform diagrams of the region B switch drive, drain-source voltage and the second filter inductor current . In region B, Therefore, the switch is in high-frequency operation, and the switch is kept off, and the grid current flows through the filter inductor before flowing into the grid, generating the second filter inductor current . As shown in Figure 7 , a switching period in region B can include the following three stages:

[0110] (1) Switching tube conduction stage

[0111] The switching tube conduction stage starts from time to time. Specifically, at time, the switch / is turned on, the second filter inductor current increases linearly from zero, to time, / is turned off, increases to the current peak . Since the switch / is continuously in the on state, the drain-source voltage . According to Kirchhoff's voltage and current law, the calculation formula of is:

[0112] (12)

[0113] In equation (12), , For the second filter inductor current peak value DC input voltage This is the grid voltage. For filter inductors and inductance value, For output capacitor and The capacitance value.

[0114] (2) Switching transistor turn-off stage

[0115] Switching transistor turn-off stage from From the moment to The moment has ended. Specifically, in time, Give / Output capacitor Charge, rise from It begins to decrease linearly, until... time, Reduce to zero. Within a time period. Inside, Keep Unchanged. According to Kirchhoff's voltage and current laws, The calculation formula is:

[0116] (13)

[0117] It should be noted that during the turn-off phase of the switching transistor... Previously, it could also include an output capacitor. Charging phase .because The duration of each phase is almost negligible; therefore, to simplify calculations, the switching cycle is not included in the calculation in some embodiments. In other embodiments, to obtain a more accurate switching cycle, [the calculation is performed]. The duration of each phase is included in the calculation of the switching cycle. Specifically, for example... Figure 8 As shown, the output capacitor Charging phase from From the moment to The moment has ended. time, Give / output capacitor charging, rises from zero to time, In this stage, approximately remains unchanged. According to Kirchhoff's voltage and current law, the calculation formula is:

[0118] (14)

[0119] (3) Third resonance stage

[0120] third resonance stage starts from time to time. Specifically, at time, the inverter enters the third resonance stage, , and form a resonance network, to 0.5 place, drops to the bottom, at which time the valley can be turned on / According to Kirchhoff's voltage and current law, the calculation formula is:

[0121] (15)

[0122] It can be understood that in the rectification mode, without external intervention, and will always be in the third resonance stage.

[0123] According to the drain-source voltage Figure 7 and the second filter inductor current in each stage in a switching cycle in the rectification mode, the change curve of the drain-source voltage is the valley, and the second filter inductor current is zero +(m+0.5) corresponding time, where is the adjustment value of the switching cycle, which is an integer greater than or equal to zero. At these times, valley voltage turn-on / can be realized. Therefore, in the rectification mode, the second switching cycle that meets the valley voltage turn-on condition. It is obvious that the minimum switching cycle is If the switching period is to be increased, the appropriate value of T can be set according to actual needs.

[0124] Please refer to Figure 9 and Figure 10 , Figure 9 is Figure 4 a set of waveform diagrams of the region C switch tube drive, drain-source voltage and the second filter inductor current , Figure 10 is Figure 4 a set of waveform diagrams of the region D switch tube drive, drain-source voltage and the first filter inductor current .

[0125] Region C is in inverter mode, Therefore, the switch tube high-frequency operation, switch tube remain off, and the grid current flows into the grid through the filter inductor , generating the second filter inductor current . In region C, the drain-source voltage and the second filter inductor current varying curves are the same as the drain-source voltage and the first filter inductor current varying curves in region A, therefore, the switching period in region C is the same as the switching period in region A.

[0126] Region D is in rectifier mode, Therefore, the switch tube high-frequency operation, switch tube remain off, and the grid current flows into the grid through the filter inductor , generating the first filter inductor current . In region D, the drain-source voltage and the first filter inductor current varying curves are the same as the drain-source voltage and the second filter inductor current varying curves in region B, therefore, the switching period in region D is the same as the switching period in region B.

[0127] So far, the calculation rule of the switching period corresponding to the four regions A, B, C, D of the grid-connected inverter in one power frequency cycle has been obtained. Among them, in regions A and C, the grid-connected inverter is in inverter mode, and the two regions correspond to the first switching period ​the calculation rule of the first switching period in which the grid-connected inverter in the inverting mode meets the zero-voltage turn-on condition the calculation rule of the first switching period in which the grid-connected inverter in the inverting mode meets the zero-voltage turn-on condition Figure 1 the calculation rule of the first switching period in which the grid-connected inverter in the inverting mode meets the zero-voltage turn-on condition the calculation rule of the first switching period in which the grid-connected inverter in the inverting mode meets the zero-voltage turn-on condition

[0128] (16)

[0129] wherein, , is the peak value of the filter inductor current in the inverting mode, is the direct-current input voltage, is the grid voltage, is the filter inductance and is the inductance value of the filter inductor, is the output capacitance and is the capacitance value of the output capacitor, is the adjustment value of the first switching period, which is an integer greater than or equal to zero.

[0130] according to the calculation rule of the first switching period , the first duty cycle of the grid-connected inverter in the inverting mode meeting the zero-voltage turn-on condition Figure 1 is obtained, which is specifically:

[0131] (17)

[0132] Figure 1 the calculation rule of the second switching period in which the grid-connected inverter in the rectifying mode meets the valley voltage turn-on condition

[0133] (18)

[0134] wherein, , is the peak value of the filter inductor current in the rectifying mode, is the adjustment value of the second switching period, which is an integer greater than or equal to zero. According to the calculation rule of the second switching period , the second duty cycle of the grid-connected inverter in the rectifying mode meeting the valley voltage turn-on condition Figure 1 is obtained, which is specifically:

[0135] (19)

[0136] Please refer to Figure 12 and​​​Figure 13 , Figure 12 is a relationship diagram between the switching frequency and the grid voltage provided by an embodiment of the application, Figure 13 is a relationship diagram between the duty cycle and the grid voltage provided by an embodiment of the application. From Figure 12 and Figure 13 It can be known that, based on the calculation rules of the switching frequency and the duty cycle of the switching tube soft switching condition according to the above formula (6) to formula (19) under the DCM control strategy, the greater the absolute value of the grid voltage of the grid-connected inverter, the smaller the switching frequency that meets the switching tube soft switching condition, and the greater the duty cycle, so that the switching on time is longer, the charging time of the inductor by the direct current input source is longer, the energy stored in the inductor is more, the inductor discharge current is greater, and the grid-connected current is greater. That is, when the absolute value of the grid voltage instantaneously rises sharply, the inductor current will increase sharply, thereby causing the output current distortion to be intensified, and in severe cases, even causing the inverter to be off-grid or the device to be damaged. To prevent current distortion, the embodiments of the application adjust the grid-connected reference current by calculating a specific adjustment coefficient, and then suppress the output current. For example, first, calculate the first adjustment coefficient based on formula (1), then calculate the grid-connected final reference current based on formula (5), then determine the switching period and the duty cycle of the switching tube that operates at high frequency based on formula (16) to formula (19), and control the switching tube to turn on at zero voltage or valley voltage according to the switching period and the duty cycle, so that when the grid voltage is in a sharp increase state, the output current can be suppressed, and the output current can be prevented from being distorted too much, thereby causing the inverter to be off-grid or the device to be damaged.

[0137] In another embodiment, the grid-connected final reference current is not only related to the first adjustment coefficient, but also introduces a second adjustment coefficient, which is specifically described as follows.

[0138] Please refer to Figure 11 , Figure 11 is a detailed flowchart of step S303 provided by an embodiment of the application, and specifically includes:

[0139] Step S303a, determining the original switching frequency of the switching tube that operates at high frequency according to the direct current input voltage, the grid voltage and the grid-connected original reference current.

[0140] According to Figure 4~Figure 10 , based on the DCM control strategy, the first switching period that meets the zero voltage turn-on condition of the grid-connected inverter in the inverter mode (the grid-connected original reference current is substituted into formula (16)) and the second switching period that meets the valley voltage turn-on condition of the grid-connected inverter in the rectifier mode (the grid-connected original reference current is substituted into formula (18)) can be calculated, and then the first original switching frequency that meets the zero voltage turn-on condition of the grid-connected inverter in the inverter mode can be calculated.The second original switching frequency of the grid-connected inverter satisfying the valley voltage turn-on condition in the rectification mode . Specifically, , .

[0141] In step S303b, the second adjustment coefficient of the grid-connected reference current is determined according to the original switching frequency of the switching tube and the cutoff frequency of the LC low-pass filter.

[0142] In an embodiment, the inductance value and the capacitance value of the LC low-pass filter are obtained, and the cutoff frequency of the LC low-pass filter is calculated based on the following formula (20):

[0143] (20)

[0144] wherein, is the cutoff frequency of the LC low-pass filter, is the inductance value of the LC low-pass filter (i.e. Figure 1 the inductance value of the inductor in the LC low-pass filter and the inductance value of the inductor in the LC low-pass filter ), is the capacitance value of the LC low-pass filter (i.e. Figure 1 the capacitance value of the capacitor in the LC low-pass filter ).

[0145] The DCM-based inverter control strategy adjusts the width of the output pulse by controlling the turn-on and turn-off time of the switching tube to achieve that the output grid-connected current is a sinusoidal signal and is synchronized with the grid. When the switching frequency of the switching tube is low, the number of pulses in each sinusoidal period is reduced, and the change of the pulse width is relatively rough, and the low-frequency harmonic content of the output grid-connected current signal is large. If the switching frequency of the switching tube of the single-phase grid-connected inverter is lower than the cutoff frequency of the LC low-pass filter, the harmonic components of the grid-connected current signal generated by the single-phase grid-connected inverter based on the DCM control strategy cannot be completely attenuated by the LC low-pass filter, and finally the harmonic content of the output grid-connected current signal is large, which increases the grid loss, reduces the power generation efficiency, interferes with the communication system, also causes the electrical equipment to heat, vibrate and generate noise, and even affects the equipment life and the stability of the grid.

[0146] In the actual single-phase grid-connected inverter circuit, there are some nonlinear elements and characteristics, such as the turn-on voltage drop of the power device, the dead time, etc. These nonlinear factors will cause the current waveform to be distorted and generate harmonics under a large grid-connected current. When the grid-connected current is small, i.e., the grid-connected current reference value is reduced, the current flowing through these nonlinear elements is relatively reduced, the influence of the nonlinear factors on the current waveform is weakened, the current waveform is closer to the ideal sinusoidal wave, and the harmonic content is reduced.

[0147] Based on the cutoff frequency of the LC low-pass filter and the switching frequency of the single-phase grid-connected inverter switching tube, a second adjustment coefficient of the grid-connected reference current is obtained:

[0148] (21)

[0149] wherein, is the second adjustment coefficient, is the original switching frequency of the switching tube, is the cutoff frequency of the LC low-pass filter. Obviously, the second adjustment coefficient When the original switching frequency of the grid-connected inverter switching tube is greater than or equal to the cutoff frequency of the low-pass filter , the coefficient (i.e. no adjustment is made to the grid-connected reference current); when the original rate of the grid-connected inverter switching tube is less than the cutoff frequency of the low-pass filter , the coefficient , and the greater the difference between the original switching frequency of the switching tube and the cutoff frequency of the low-pass filter , the smaller the coefficient , thereby adaptively reducing the grid-connected reference current, reducing the harmonic content, avoiding distortion of the output current, and improving the grid efficiency.

[0150] In step S303c, based on the first adjustment coefficient, the second adjustment coefficient and the grid-connected original reference current, a grid-connected final reference current is obtained.

[0151] In an embodiment, step S303c specifically comprises: first, based on the first adjustment coefficient and the second adjustment coefficient, a comprehensive adjustment coefficient is obtained; and then, based on the comprehensive adjustment coefficient and the grid-connected original reference current, the grid-connected final reference current is obtained. Specifically, the calculation formula of the comprehensive adjustment coefficient is:

[0152] (22)

[0153] wherein, , is a weight coefficient, .

[0154] The calculation formula of the grid-connected final reference current is:

[0155] (23)

[0156] wherein, is the grid-connected original reference current, is the grid-connected final reference current.

[0157] Similarly, the integrated adjustment coefficient is calculated based on the formula (21) and the formula (22) first, the grid-connected final reference current is calculated based on the formula (23) then, the switching period and the duty cycle of the switching tube are determined according to the formula (16)-formula (19), and the switching tube is controlled to be turned on at zero voltage or valley voltage according to the switching period and the duty cycle, and when the grid voltage is in the surge state, the suppressed grid-connected current is output to realize the power synchronization with the public grid, feed the power into the grid, and effectively solve the problem that the control strategy based on the DCM theory calculation cannot adapt to the sudden change of the grid voltage.

[0158] Please refer to Figure 14a and Figure 14b , which are oscilloscope measurement graphs of the grid voltage surge moment using the traditional control method and the control method of the application respectively. As shown in Figure 14a , the yellow waveform is the grid voltage, and the green waveform is the grid-connected current. At the grid voltage surge moment (t0 moment), the grid-connected current output by the traditional control method has a large peak, triggers overcurrent, and the system stops. As shown in Figure 14b , the yellow waveform is the grid voltage, and the green waveform is the grid-connected current. At the grid voltage surge moment (t0 moment), the control method of the application can quickly respond and effectively suppress the grid-connected current peak, and does not trigger overcurrent.

[0159] The grid-connected inverter control method provided by the embodiment of the application first acquires the DC input voltage and the grid voltage, determines the first adjustment coefficient of the grid-connected reference current based on the DC input voltage and the grid voltage, then determines the grid-connected original reference current according to the grid voltage and the preset grid-connected power, obtains the grid-connected final reference current based on the first adjustment coefficient and the grid-connected original reference current, and then determines the switching tube of high-frequency action and the switching period and the duty cycle of the switching tube based on the DC input voltage, the grid voltage and the grid-connected final reference current, and controls the switching tube to be turned on and turned off based on the switching period and the duty cycle of the switching tube. The method of the application does not need to detect the abnormal working condition of the grid, can simultaneously adapt to the adjustment of the output current when the grid voltage is in the steady state or the surge state through the calculation of the specific adjustment coefficient, can suppress the output current when the grid voltage is in the surge state, and avoid the large distortion of the output current, which causes the inverter to be off-grid or the device to be damaged.

[0160] Finally, it should be noted that: the above examples are only used to illustrate the technical solutions of the present application, but not limited to them; under the idea of the present application, the technical features of the above examples or different examples can also be combined, the steps can be implemented in any order, and there are many other changes of different aspects of the present application as described above, which are not provided in details for simplicity; although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing examples, or make equivalent replacement to part of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A grid-connected inverter control method, characterized by, The grid-connected inverter comprises an LC low-pass filter, and the method comprises: obtaining a direct-current input voltage and a grid voltage, determining a first adjustment coefficient of a grid-connected reference current based on the direct-current input voltage and the grid voltage; determining a grid-connected original reference current according to the grid voltage and a preset grid-connected power; determining an original switching frequency of a switching tube according to the direct-current input voltage, the grid voltage and the grid-connected original reference current; determining a second adjustment coefficient of the grid-connected reference current according to the original switching frequency of the switching tube and a cutoff frequency of the LC low-pass filter; obtaining a comprehensive adjustment coefficient based on the first adjustment coefficient and the second adjustment coefficient; obtaining a final grid-connected reference current based on the comprehensive adjustment coefficient and the grid-connected original reference current; determining a switching tube of high-frequency action and a switching period and a duty cycle of the switching tube based on the direct-current input voltage, the grid voltage and the final grid-connected reference current, and controlling the switching tube to turn on and turn off based on the switching period and the duty cycle of the switching tube; a calculation formula of the first adjustment coefficient is: wherein is the first regulation factor, is the DC input voltage, is the grid voltage, is the DC input voltage average, is the grid voltage effective value; a calculation formula of the second adjustment coefficient is: wherein, is the second adjustment coefficient, is the original switching frequency of the switching transistor, is the cut-off frequency of the LC low-pass filter.

2. The method of claim 1, wherein, the cutoff frequency of the LC low-pass filter is calculated based on an inductance value and a capacitance value of the LC low-pass filter.

3. The method according to claim 1 or 2, characterized in that, The determination of the original switching frequency of the switching tube according to the direct-current input voltage, the grid voltage and the grid-connected original reference current comprises: determining a working mode in which the grid-connected inverter currently stays based on the polarity of the direct-current input voltage and the grid-connected original reference current, the working mode comprising an inverting mode and a rectifying mode; determining the original switching frequency of the switching tube based on a period calculation rule corresponding to a zero-voltage turn-on condition satisfied by the direct-current input voltage, the grid voltage and the working mode.

4. The method of claim 3, wherein, The grid-connected inverter further comprises a direct-current voltage input source, a switching tube and a body diode connected in parallel to the switching tube and an output capacitor , a filter inductor and connected to the bridge arm where the switching tube and the filter capacitor are located , a filter inductor and connected to the bridge arm where the switching tube and the filter capacitor are located , and a power grid, wherein a calculation rule of a first switching period of the grid-connected inverter satisfying a zero-voltage turn-on condition in an inverting mode is The grid-connected inverter meets the valley voltage turn-on condition in the second switch period in the rectification mode The calculation rule is: wherein, , , is the peak value of the filtered inductor current in the inversion mode, is the peak value of the filtered inductor current in the rectification mode, is the DC input voltage, is the grid voltage, is the inductance value of the filtered inductor and , is the capacitance value of the output capacitor and , is the grid-connected reference current, is the adjustment value of the first switching period, is the adjustment value of the second switching period, and is an integer greater than or equal to zero.

5. A grid-connected inverter, characterized by The grid-connected inverter comprises a controller and an LC low-pass filter, the controller is configured to obtain a direct-current input voltage and a grid voltage, determine a first adjustment coefficient of a grid-connected reference current based on the direct-current input voltage and the grid voltage, determine a grid-connected original reference current according to the grid voltage and a preset grid-connected power, and determine an original switching frequency of a switching tube according to the direct-current input voltage, the grid voltage and the grid-connected original reference current; determine a second adjustment coefficient of the grid-connected reference current according to the original switching frequency of the switching tube and a cutoff frequency of the LC low-pass filter, obtain a comprehensive adjustment coefficient based on the first adjustment coefficient and the second adjustment coefficient, obtain a final grid-connected reference current based on the comprehensive adjustment coefficient and the grid-connected original reference current, determine a switching tube of high-frequency action and a switching period and a duty cycle of the switching tube based on the direct-current input voltage, the grid voltage and the final grid-connected reference current, and control the switching tube to turn on and turn off based on the switching period and the duty cycle of the switching tube; a calculation formula of the first adjustment coefficient is: wherein is the first regulation factor, is the DC input voltage, is the grid voltage, is the DC input voltage average value, is the grid voltage effective value, a calculation formula of the second adjustment coefficient is: wherein, is the second adjustment coefficient, is the original switching frequency of the switching transistor, is the cut-off frequency of the LC low-pass filter.

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