Photovoltaic grid-connected inverter and alternating current side starting method of photovoltaic grid-connected inverter

Abstract

The invention relates to a photovoltaic grid-connected inverter and an alternating current side starting method of the photovoltaic grid-connected inverter, and the inverter comprises a bus capacitor, a DC / AC inverter circuit, a control unit and a pre-charging circuit. The input end of the DC / AC inverter circuit is connected with the bus capacitor, and the output end of the DC / AC inverter circuit is connected with a power grid; the input end of the pre-charging circuit is connected with a power grid, the output end of the pre-charging circuit is connected with the bus capacitor, and the pre-charging circuit is used for taking electricity from the midpoint of the grid-connected relay to charge the bus capacitor; the bus capacitor is used for starting the DC / AC inverter circuit after charging is completed; the control unit is used for controlling work of the pre-charging circuit. According to the invention, the AC side starting of the inverter can be realized, the starting process is safer, and the work is stable and reliable.

Description

Technical Field

[0001] The present invention relates to the technical field of photovoltaic equipment, and in particular to a photovoltaic grid-connected inverter and an AC side starting method of the photovoltaic grid-connected inverter. Background Art

[0002] At present, the power grid often requires photovoltaic grid-connected inverters to generate reactive power for power compensation at night. When the photovoltaic grid-connected inverter is not started before night, due to the lack of sunlight at night, the solar panels cannot supply power to the photovoltaic inverter grid-connected unit, and the photovoltaic grid-connected inverter cannot be started, so reactive compensation cannot be completed. At the same time, considering that before night, if the photovoltaic grid-connected inverter has been connected to the grid and has not been disconnected from the grid, but when the photovoltaic grid-connected inverter has a fault alarm at night, the photovoltaic grid-connected inverter will suspend work. After the fault is removed, the photovoltaic grid-connected power generation system cannot be started again, which affects the scheduling work. Therefore, the photovoltaic grid-connected inverter cannot be started when there is no sunlight, which may affect the scheduling work of the photovoltaic grid-connected power generation system when there is no sunlight.

[0003] In order to solve the above problems, the existing technology uses the rectification function of the inverter circuit to realize the charging of the bus capacitor, and reduces the impact current by connecting a current limiting circuit in parallel to the grid-connected switch. However, this solution can only pre-charge the bus voltage to 1.44 times the grid line voltage, which often cannot reach the DC bus voltage required for grid connection. Software control of the inverter circuit is required to work in a controllable rectification mode to increase the bus voltage to the voltage required for grid connection, which makes the control more complicated. There is also a solution to add an additional rectification circuit to convert the grid voltage into direct current, and then increase the bus voltage to the target voltage through an isolated DC / DC circuit, but it requires adding more devices and coordinating corresponding control strategies. Summary of the invention

[0004] A main purpose of the present invention is to overcome at least one of the above-mentioned defects, and to provide a photovoltaic grid-connected inverter and an AC side starting method of the photovoltaic grid-connected inverter, which can realize the AC side starting of the inverter, and the starting process is safer, and the operation is stable and reliable.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is: The present invention provides a photovoltaic grid-connected inverter, which includes a bus capacitor, a DC / AC inverter circuit, a control unit, a pre-charging circuit and a current limiting resistor; The external DC input is connected to the input end of the DC / AC inverter circuit via a DC bus, the bus capacitor is arranged between the DC input and the DC / AC inverter circuit, the bus capacitor is connected in parallel between the DC buses, and the output end of the DC / AC inverter circuit is connected to the power grid; The input end of the pre-charging circuit is connected to the current limiting resistor and then connected to the power grid, the output end of the pre-charging circuit is connected to the bus capacitor, and the pre-charging circuit is used to take power from the power grid to charge the bus capacitor; The bus capacitor is used to start the DC / AC inverter circuit after charging is completed; The control unit is used to control the operation of the pre-charging circuit, and control the DC / AC inverter circuit to output alternating current for grid connection after the bus capacitor is fully charged.

[0006] According to one embodiment of the present invention, the output end of the DC / AC inverter circuit is connected to the power grid via a grid-connected switch, the grid-connected switch includes at least one group of grid-connected relays, each group of grid-connected relays includes at least two relays connected in series, the pre-charging circuit is used to draw power from the grid-connected relay to charge the bus capacitor, and there is at least one relay between the power-drawing position and the DC / AC inverter circuit.

[0007] According to one embodiment of the present invention, the bus capacitor is divided into a positive bus capacitor and a negative bus capacitor, the midpoint of the bus capacitor is between the positive bus capacitor and the negative bus capacitor, the positive bus capacitor and the negative bus capacitor each have one capacitor or at least two capacitors, the at least two capacitors are connected in series or in parallel, and the positive bus capacitor and the negative bus capacitor are connected in series and then connected in parallel between the positive and negative DC buses of the inverter.

[0008] According to one embodiment of the present invention, the output end of the pre-charging circuit is divided into three connection points, which are respectively connected to the positive electrode, the negative electrode and the midpoint of the bus capacitor, and the pre-charging circuit is used to charge the positive bus capacitor and the negative bus capacitor in the positive and negative half cycles of the power grid.

[0009] According to one embodiment of the present invention, the inverter is a single-phase inverter, a split-phase inverter or a three-phase inverter, the grid-connected switch includes at least two groups of grid-connected relays, and a group of grid-connected relays are respectively connected to each phase AC bus of the inverter, and the input end of the pre-charging circuit is connected to any two-phase output ends of the DC / AC inverter circuit through a current limiting resistor.

[0010] According to one embodiment of the present invention, the pre-charging circuit includes a slow-start switch K1, a slow-start switch K2, a first diode D1 and a second diode D2, the slow-start switch K1 and the slow-start switch K2 are respectively connected in series with a current limiting resistor to form a first branch and a second branch, one end of the first branch is connected to one phase of the power grid, and the other end is connected to the positive electrode of the first diode D1, the negative electrode of the first diode D1 is connected to the positive electrode of the bus capacitor, the positive electrode of the second diode D2 is connected to the negative electrode of the bus capacitor, one end of the second branch is connected to another phase of the power grid, and the other end is connected to the midpoint of the bus capacitor; The first diode D1 and the second diode D2 are used to charge the positive bus capacitor and the negative bus capacitor of the bus capacitor respectively in the positive and negative half cycles of the power grid.

[0011] According to one embodiment of the present invention, the pre-charging circuit includes a slow-start switch K2, a first diode D1, a second diode D2, a protective resistor and a pre-charging switch, the slow-start switch K2 is connected in series with a current-limiting resistor to form a first branch, one end of the second branch is connected to one phase of a power grid, and the other end is connected to the positive electrode of the first diode D1, the negative electrode of the first diode D1 is connected to the positive electrode of the bus capacitor, the positive electrode of the second diode D2 is connected to the negative electrode of the bus capacitor, the protective resistor and the pre-charging switch are connected in series to form a second branch, and the second branch is connected in parallel to the two ends of another phase grid-connected relay.

[0012] According to one embodiment of the present invention, the pre-charging circuit includes a slow-start switch K2, one end of the circuit after the slow-start switch K2 and the current-limiting resistor are connected in series is connected to one phase of the power grid, and the other end is connected to the midpoint of the bus capacitor to form a second circuit, and the bridge arm in the DC / AC inverter circuit connected to the other phase of the power grid is used as the first circuit, the bridge arm includes two switching tubes connected in series, and the bridge arm is connected in parallel with the bus capacitor, and is used to charge the positive bus capacitor and the negative bus capacitor of the bus capacitor in the positive and negative half cycles of the power grid, respectively.

[0013] According to one embodiment of the present invention, the first loop further includes a protection resistor and a pre-charge switch, and a circuit formed by the series connection of the protection resistor and the pre-charge switch is connected in parallel to two ends of another phase grid-connected relay.

[0014] According to one embodiment of the present invention, the pre-charging circuit includes a first protection resistor, a first pre-charging switch, a second protection resistor and a second pre-charging switch, and any phase of the DC / AC inverter circuit includes a first bridge arm and a second bridge arm, the first bridge arm is two switch tubes connected in series in parallel to the bus capacitor, the second bridge arm is two switch tubes connected in reverse series, one end of the second bridge arm is connected to the midpoint of the bus, and the other end is connected to the midpoint of the first bridge arm, one end of the first circuit formed by the first protection resistor and the first pre-charging switch being connected in series is connected to a grid-connected relay of one phase, and the other end is connected to the midpoint of the first bridge arm, and one end of the second circuit formed by the second protection resistor and the second pre-charging switch being connected in series is connected to the grid-connected relay of another phase, and the other end is connected to the second bridge arm.

[0015] According to one embodiment of the present invention, an AC auxiliary power supply is further included, wherein an input end of the AC auxiliary power supply is connected to a power grid, and an output end of the AC auxiliary power supply is connected to the control unit for supplying power to the control unit.

[0016] In particular, the present invention also provides an AC side starting method based on the photovoltaic grid-connected inverter as described above, which comprises the following working steps: The startup state of the inverter is judged. If it is AC startup, the slow-start switch in the pre-charging circuit is closed to charge the bus capacitor by connecting to the power grid. Otherwise, the inverter is directly started and connected to the grid. The voltage of the bus capacitor is detected. If the bus capacitor reaches the voltage required for grid connection, the slow-start switch in the pre-charging circuit is disconnected. After the disconnection is completed, the grid connection is started. Otherwise, the bus capacitor is kept charged. In the process of starting and stopping the pre-charging circuit, the current limiting resistor or the protection resistor is used to perform current limiting protection on the charging of the bus capacitor and to limit the common mode current impact.

[0017] Compared with the prior art, the advantages and beneficial effects of the photovoltaic grid-connected inverter and the AC side starting method of the photovoltaic grid-connected inverter in the patent application of the present invention are: The photovoltaic inverter of the present application optimizes the circuit structure, takes AC power from the grid side, and then charges the bus capacitor through a pre-charging circuit. When the bus capacitor voltage reaches the voltage required for grid connection, the DC / AC inverter circuit is started, thereby completing the startup and grid connection of the photovoltaic grid-connected inverter at night or in the absence of sunlight, saving cost investment, and ensuring higher equipment working stability and reliability.

[0018] Furthermore, the present application utilizes the original grid-connected relay close to the grid side in the photovoltaic grid-connected inverter in combination with a relay control strategy to add a layer of protection for the bus capacitor charging circuit, thereby avoiding directly taking AC power from the AC power grid, thereby preventing the AC power grid from charging the bus capacitor and causing bus capacitor overvoltage when the slow-start switch (which can be a relay or a power switching device) in the newly added pre-charging circuit is stuck and cannot be disconnected, that is, the starting relay is stuck and cannot be disconnected. If the bus capacitor voltage is high, the grid-connected relay can be disconnected urgently to cut off the connection between the pre-charging circuit and the AC power grid to complete the protection.

[0019] The slow-start circuit structure designed in the present application utilizes the half-wave rectification principle to charge the half-bus capacitors separately, for example, the positive bus capacitor is charged in the positive half-cycle of the grid voltage, and the negative bus capacitor is charged in the negative half-cycle of the grid voltage, so as to achieve the voltage doubling effect, and the bus voltage can be raised to 2 times the line voltage peak value, while avoiding complex rectification circuits and isolated DC / DC circuits, making the control method simpler. Moreover, since the pre-charging circuit can charge the bus voltage to 2 times the grid line voltage peak value at most, the control unit can control the inverter circuit to output AC power that is consistent with the grid voltage amplitude and phase, so that when the grid switch is attracted, there is no voltage difference between the inverter circuit output and the grid voltage, so there is no impact current, and the soft start of the inverter is realized, which improves the working safety and reliability of the inverter.

[0020] When charging the bus capacitor, the present application is compatible with the strategy mechanism of bus capacitor overvoltage protection. When the bus capacitor voltage is too high, the control unit can send a control signal to urgently cut off the grid-connected relay or cut off the switch or relay in the pre-charging circuit.

[0021] In addition, the photovoltaic grid-connected inverter of the present application has good compatibility and can be applied to three-phase three-wire, three-phase four-wire, split-phase, and single-phase systems, and has strong adaptability. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Hereinafter, some specific embodiments of the present invention will be described in detail in an exemplary and non-limiting manner with reference to the accompanying drawings. The same reference numerals in the accompanying drawings indicate the same or similar components or parts. It should be understood by those skilled in the art that these drawings are not necessarily drawn to scale. In the accompanying drawings: Figure 1 is a schematic diagram of the framework structure after the photovoltaic grid-connected inverter in Embodiment 1 of the present invention is connected to the DC input and the AC power grid; Figure 2 is a schematic diagram of the connection structure between the pre-charging circuit of the photovoltaic grid-connected inverter and the power grid according to Embodiment 1 of the present invention; Figure 31 is a schematic diagram of a circuit structure after the three-phase photovoltaic grid-connected inverter in Embodiment 2 of the present invention is connected to an AC power grid, wherein the power supply of the pre-charging circuit is located at the grid-connected switch; Figure 4 is a schematic diagram of the framework structure after the single-phase photovoltaic grid-connected inverter in Embodiment 3 of the present invention is connected to the AC power grid; Figure 5 is a schematic diagram of a circuit structure after a three-phase photovoltaic grid-connected inverter is connected to an AC power grid according to Embodiment 4 of the present invention; Figure 6 is a schematic diagram of a circuit structure after the three-phase photovoltaic grid-connected inverter in Embodiment 5 of the present invention is connected to an AC power grid; Figure 7 is a schematic diagram of a circuit structure after the three-phase photovoltaic grid-connected inverter in Embodiment 6 of the present invention is connected to an AC power grid; Figure 8 is a schematic diagram of the working process of the AC side starting method of the photovoltaic grid-connected inverter in Embodiment 7 of the present invention; Fig. 9 1 is a schematic diagram of the working process of the AC side starting method of the photovoltaic grid-connected inverter in Example 8 of the present invention.

[0023] The reference numerals are as follows: 101. DC input, 102. bus capacitor, 103. DC / AC inverter circuit, 104. grid-connected switch, 105. power grid, 106. AC auxiliary power supply, 107. control unit, 108. pre-charging circuit, 109. control unit current limiting resistor. DETAILED DESCRIPTION

[0024] The technical solution of the present invention will be described clearly and completely below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0025] In addition, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0026] Embodiment 1: like Figure 1 As shown, this embodiment describes a photovoltaic grid-connected inverter, which includes a bus capacitor 102, a DC / AC inverter circuit 103, a control unit 107, a pre-charging circuit 108 and a current limiting resistor 109; The external DC input 101 is connected to the input end of the DC / AC inverter circuit 103 via a DC bus, the bus capacitor 102 is provided between the DC input 101 and the DC / AC inverter circuit 103, and the output end of the DC / AC inverter circuit 103 is connected to the AC grid 105; The input end of the pre-charging circuit 108 is connected to the current limiting resistor 109 and then connected to the power grid. The output end of the pre-charging circuit 108 is connected to the bus capacitor 102. The pre-charging circuit 108 is used to draw power from the AC power grid 105 to charge the bus capacitor 102. The bus capacitor 102 is used to start the DC / AC inverter circuit 103 after charging is completed; The control unit 107 is used to control the operation of the pre-charging circuit 108 , and control the DC / AC inverter circuit 103 to output alternating current for grid connection after the bus capacitor 102 is fully charged.

[0027] When the photovoltaic grid-connected inverter of this embodiment is put into use, the solar panel and the DC / DC boost circuit can form the DC input 101, and the DC / DC boost circuit boosts the DC power output by the solar panel. The DC input 101 is connected to the bus capacitor 102; the AC power grid 105 is connected to the AC side of the photovoltaic grid-connected inverter.

[0028] It is not difficult to find that this embodiment takes AC power from any two phases of the inverter and then charges the bus capacitor 102 through the pre-charging circuit 108. When the bus capacitor voltage reaches the voltage required for grid connection, the DC / AC inverter circuit 103 is started, thereby completing the startup and grid connection of the photovoltaic grid-connected inverter at night or in the absence of sunlight.

[0029] The photovoltaic grid-connected inverter of this embodiment can use the half-wave rectification principle to charge the half-bus capacitors separately. For example, the upper half-bus capacitor (positive bus capacitor) is charged in the positive half-cycle of the grid voltage, and the lower half-bus capacitor (negative bus capacitor) is charged in the negative half-cycle of the grid voltage, so as to achieve the voltage doubling effect, and can raise the bus voltage to twice the line voltage peak value. At the same time, it avoids complex rectification circuits and isolated DC / DC circuits, making the control method simpler.

[0030] In one embodiment, the inverter also includes an AC auxiliary power supply 106, the input end of which is connected to the power grid, and the output end is connected to the control unit 107, for supplying power to the control unit 107. The control unit 107 is used to control the operation of the pre-charging circuit, and control the DC / AC inverter circuit to output AC power for grid connection after the bus capacitor is charged.

[0031] The bus capacitor 102 may be composed of two or more capacitors connected in series, and each capacitor in the bus capacitor 102 is connected in series and then connected in parallel between the positive and negative DC buses of the inverter. The bus capacitor 102 may be divided into a positive bus capacitor and a negative bus capacitor, such as Figure 1 In the bus capacitor 102 shown, the upper DC bus is the positive DC bus, and the lower DC bus is the negative DC bus. The capacitor located in the upper part connected to the positive DC bus is defined as the positive bus capacitor, and the capacitor located in the lower part connected to the negative DC bus is defined as the negative bus capacitor. The connection point between the positive bus capacitor and the negative bus capacitor is the midpoint of the bus capacitor. The positive bus capacitor and the negative bus capacitor each have at least one capacitor, and generally speaking, the capacitance values ​​of the positive bus capacitor and the negative bus capacitor in the two circuits are equal. Of course, two or more capacitors can also be respectively set in the positive bus capacitor or the negative bus capacitor, and the two or more capacitors constituting the positive bus capacitor or the negative bus capacitor can be connected in series and / or in parallel and then connected in series to form the bus capacitor 102, but generally speaking, it is necessary to ensure that the capacitance values ​​of the positive bus capacitor and the negative bus capacitor in the two circuits are equal.

[0032] like Figure 1 As shown, the output end of the pre-charging circuit 108 is divided into three connection points, which are respectively connected to the positive electrode, the negative electrode and the midpoint of the bus capacitor 102. The pre-charging circuit 108 is used to charge the positive bus capacitor and the negative bus capacitor in the positive and negative half cycles of the power grid.

[0033] like Figure 1 and Figure 2 As shown, specifically, the pre-charging circuit 108 includes a slow-start switch K1, a slow-start switch K2, a first diode D1 and a second diode D2, and the current-limiting resistor 109 includes a current-limiting resistor R1 and a current-limiting resistor R2. The slow-start switch K1 and the slow-start switch K2 are respectively connected in series with a current-limiting resistor to form a first branch and a second branch. The first branch is connected in series with the current-limiting resistor R1, that is, the slow-start switch K1 and the current-limiting resistor R1 are connected in series, and the second branch is connected in series with the current-limiting resistor R2, that is, the slow-start switch K2 and the current-limiting resistor R2 are connected in series, and the first branch One end of the second branch is connected to one phase of the power grid, and the other end is connected to the positive electrode of the first diode D1, the negative electrode of the first diode D1 is connected to the positive electrode of the positive bus capacitor (the positive electrode is the end of the positive bus capacitor connected to the positive DC bus), the positive electrode of the second diode D2 is connected to the negative electrode of the bus capacitor 102 (the negative electrode is the end of the negative bus capacitor connected to the negative DC bus), one end of the second branch is connected to another phase of the power grid, and the other end is connected to the midpoint of the bus capacitor (the connection point between the positive bus capacitor and the negative bus capacitor); The first diode D1 and the second diode D2 are used to charge the positive bus capacitor and the negative bus capacitor of the bus capacitor 102 respectively in the positive and negative half cycles of the power grid.

[0034] The photovoltaic grid-connected inverter of this embodiment charges the upper bus capacitor (positive bus capacitor) through the positive half cycle of the grid voltage, and charges the lower bus capacitor (negative bus capacitor) through the negative half cycle of the grid voltage, so as to achieve the voltage doubling effect, and can raise the bus voltage to 2 times the line voltage peak value, while avoiding complex rectification circuits and isolated DC / DC circuits, making the control method simpler. Moreover, since the pre-charging circuit 108 can charge the bus voltage to 2 times the grid line voltage peak value at most, the control unit 107 can control the inverter circuit to output AC power consistent with the grid voltage amplitude and phase, so that when the inverter circuit is connected to the grid, there is no voltage difference between the inverter circuit output and the grid voltage, so there is no impact current, and the soft start of the inverter is realized, which improves the working safety and reliability of the inverter.

[0035] Embodiment 2: The photovoltaic grid-connected inverter of this embodiment is improved on the basis of the embodiment 1, in that the power-taking point where the pre-charging circuit 108 is connected to the AC power grid 105 is set at the grid-connected switch 104 .

[0036] like Figure 3 As shown, the grid-connected switch 104 includes at least one group of grid-connected relays, each group of grid-connected relays includes two relays connected in series; the input end of the pre-charging circuit 108 is connected to the connection point between the two relays in the grid-connected relay, the output end of the pre-charging circuit 108 is connected to the bus capacitor 102, the pre-charging circuit 108 is used to draw power from the midpoint of the grid-connected relay to charge the bus capacitor 102, and the control unit 107 is used to control the operation of the grid-connected switch 104 and the pre-charging circuit 108.

[0037] The photovoltaic grid-connected inverter can be a single-phase inverter, a split-phase inverter or a three-phase inverter. The grid-connected switch 104 includes at least two groups of grid-connected relays, and a group of grid-connected relays is respectively connected to each phase AC bus of the inverter. The pre-charging circuit 108 is connected to any two groups of grid-connected relays in the grid-connected switch 104.

[0038] Specific as Figure 3 As shown, a three-phase inverter of a DC / AC inverter circuit 103 with a two-level topology structure is specifically designed.

[0039] Each phase bridge arm in the DC / AC inverter circuit 103 is composed of two switch tubes, each phase is connected to the grid switch 104 through an LC filter circuit, and the three-phase A, B, and C phase lines are respectively connected in series with the grid relay KA2, the grid relay KB2, and the grid relay KC2 on the connection path connected to each phase of the power grid. One end of the first branch is connected to the grid relay KB2 of the B phase, and one end of the second branch is connected to the grid relay KC2 of the C phase.

[0040] In the absence of DC input, a power-on command is issued, and the control unit 107 detects the bus capacitor voltage. If the bus capacitor voltage is less than the target voltage, the control unit 107 controls the absorbing grid relays KB2 and KC2, and then absorbs the slow-start switch K1 and the slow-start switch K2 of the pre-charging circuit 108. The AC grid 105 charges the bus capacitors C1 and C2 through the pre-charging circuit 108. When the voltages of C1 and C2 reach the target voltage, the slow-start switch K1 and the slow-start switch K2 of the pre-charging circuit 108 are disconnected. The control unit 107 can also disconnect KB2, KC2 and the slow-start switch K1 and the slow-start switch K2 when the bus capacitors C1 and C2 are over-voltage.

[0041] When the bus capacitors C1 and C2 reach the target voltage, the control unit 107 can control the inverter to perform insulation impedance detection to prevent the DC input 101 side (PV side) from having too low impedance to ground, and to prevent a large common mode current from passing through the inverter after the inverter is running, causing damage to the components.

[0042] The control unit 107 can generate a control signal for controlling the on and off of each switch tube in the DC / AC inverter circuit 103 based on the grid voltage sampling value. The control unit 107 mainly calculates the duty cycle of the switch tube control signal through the grid voltage sampling value and the bus capacitor voltage value at this time, and obtains the pulse width modulation signal corresponding to the switch tube of each phase bridge arm, so that the DC / AC inverter circuit 103 outputs a sinusoidal voltage consistent with the amplitude and phase of the AC grid 105, and finally, when the grid-connected switch 104 is closed, no impact current is generated, and the AC soft start of the AC grid-connected inverter is realized.

[0043] In this embodiment, AC power is taken from the midpoints of two relays in the grid-connected relays of any two phases of the inverter, and then the bus capacitor 102 is charged through the pre-charging circuit 108. When the bus capacitor voltage reaches the voltage required for grid connection, the DC / AC inverter circuit 103 is started, thereby completing the startup and grid connection of the photovoltaic grid-connected inverter at night or in the absence of sunlight.

[0044] That is to say, the present application utilizes the original grid-connected relay close to the grid side in the photovoltaic grid-connected inverter in combination with the relay control strategy to add a layer of protection for the bus capacitor charging circuit, thereby avoiding taking AC power directly from the AC power grid 105, thereby preventing the AC power grid 105 from continuously charging the bus capacitor and causing the bus capacitor overvoltage problem when the slow-start switch in the newly added pre-charging circuit 108 becomes stuck and cannot be disconnected, that is, the starting relay becomes stuck and cannot be disconnected. If the bus capacitor voltage is high, the grid-connected relay can be disconnected urgently to cut off the connection between the pre-charging circuit 108 and the AC power grid 105 to complete the protection.

[0045] Embodiment 3: The photovoltaic grid-connected inverter of this embodiment has a substantially identical structure to the photovoltaic grid-connected inverter described in Embodiment 2, except that this embodiment is specifically designed for a DC / AC inverter circuit 103 of a single-phase inverter.

[0046] like Figure 4 As shown, the bridge arms corresponding to the live wire and the neutral wire in the DC / AC inverter circuit 103 are respectively composed of two switch tubes, and are then connected to the grid-connected switch 104 through an LC filter circuit. The live wire and the neutral wire are respectively connected in series with a grid-connected relay KA2 and a grid-connected relay KB2 on the connection path connected to the power grid. One end of the first branch is connected to the grid-connected relay KA2 of the live wire, and one end of the second branch is connected to the grid-connected relay KB2 of the neutral wire.

[0047] Embodiment 4: The photovoltaic grid-connected inverter of this embodiment is further limited on the basis of Embodiment 2, specifically in the circuit connection relationship between the pre-charging circuit 108 and the bus capacitor 102, and the power devices in the DC / AC inverter circuit are used to construct the first loop and the second loop. The inverter in this embodiment is a three-phase inverter.

[0048] Specifically, if Figure 5As shown, the pre-charging circuit 108 includes a slow-start switch K2 and a current-limiting resistor R2. One end of the circuit after the slow-start switch K2 and the current-limiting resistor R2 are connected in series is connected to a grid-connected relay KC2, and the other end is connected to the midpoint of the bus capacitor to form a second circuit. The half-wave circuit in the DC / AC inverter circuit 103 connected to another grid-connected relay KA2 is used as the first circuit. The first circuit is connected in parallel with the bus capacitor 102, and is used to charge the positive bus capacitor and the negative bus capacitor of the bus capacitor 102 in the positive and negative half cycles of the power grid. The first circuit also includes a protection resistor R3 and a pre-charging switch K3. One end of the circuit formed by the protection resistor R3 and the pre-charging switch K3 is connected in series to the grid-connected relay KA2 close to the power grid, and the other end is connected to the half-wave circuit. The role played by the protection resistor is basically the same as that played by the aforementioned current-limiting resistor, and it plays the role of current limiting protection for charging the bus capacitor and limiting the impact of common-mode current.

[0049] The half-wave circuit includes two switch tubes connected in series in sequence, and the other end of the circuit formed by the protection resistor and the pre-charge switch being connected in series is connected between the two switch tubes.

[0050] It can be understood that the photovoltaic grid-connected inverter of this embodiment charges the bus capacitor 102 by rectifying the mottled circuit formed by the power devices in the DC / AC inverter circuit 103. The half-wave circuit formed by the two switch tubes corresponding to the A-phase bridge arm of the three-phase inverter of this embodiment is selected.

[0051] In the absence of DC input, the inverter issues a start-up command, and the control unit 107 detects the bus capacitor voltage. If the bus capacitor voltage is less than the target voltage, the control unit 107 controls the grid relays Ka1 and Kc1, and then closes the slow-start switch K2 of the pre-charging circuit 108. The AC grid 105 charges the bus capacitors C1 and C2 through the half-wave circuit of the A-phase bridge arm. When the voltages of C1 and C2 reach the target voltage, the slow-start switch K2 of the pre-charging circuit 108 is disconnected. The control unit 107 can also disconnect KA2, KC2 and the slow-start switch K2 when the bus capacitors C1 and C2 are over-voltage. The presence of the protective resistor R3 and the pre-charging switch K3 can further reduce the impact current when the AC grid 105 is connected to charge the bus capacitor 102.

[0052] Embodiment 5: This embodiment is an improvement made on the basis of the photovoltaic grid-connected inverter described in Embodiment 4.

[0053] like Figure 6As shown, the pre-charging circuit 108 includes a slow-start switch K1 and a current-limiting resistor R1, and the first circuit formed by the pre-charging circuit 108 is connected to the circuit formed by the diode D1 and the diode D2. The diodes D1 and D2 connected in reverse parallel are connected to the positive and negative ends of the bus capacitor 102, and the bridge arm cross tube in the DC / AC inverter circuit 103 connected to another grid-connected relay KA2 is used as the second circuit, and the second circuit is connected to the midpoint of the bus capacitor, thereby forming a circuit for charging the bus capacitor.

[0054] Embodiment 6: The photovoltaic grid-connected inverter of the present embodiment is further defined on the basis of Example 2, specifically in the circuit connection relationship between the pre-charging circuit 108 and the bus capacitor 102, and the freewheeling diode of the one-phase bridge arm in the DC / AC inverter circuit and the cross tube of the other phase bridge arm are used to construct a circuit for charging the bus capacitor. The pre-charging circuit includes a first protection resistor, a first pre-charging switch, a second protection resistor and a second pre-charging switch, and any phase of the DC / AC inverter circuit includes a first bridge arm and a second bridge arm, the first bridge arm is two sequentially connected in series switch tubes connected in parallel with the bus capacitor, the second bridge arm is two reversely connected in series switch tubes, one end of the second bridge arm is connected to the bus midpoint, and the other end is connected to the midpoint of the first bridge arm, and the first protection resistor is connected in series with the first pre-charging switch. The first circuit formed by connecting one end to the grid-connected relay of one phase is connected, and the other end is connected to the midpoint of the first bridge arm, and the second protection resistor is connected in series with the second pre-charging switch. The second circuit formed by connecting one end to the grid-connected relay of another phase is connected, and the other end is connected to the second bridge arm. The inverter in this embodiment is a three-phase inverter.

[0055] Specifically, if Figure 7 As shown, the pre-charging circuit 108 includes a first protection resistor R3, a first pre-charging switch K3, a second protection resistor R4 and a second pre-charging switch K4. The freewheeling diodes of the two switch tubes Qa1 and Qa4 of the A-phase bridge arm in the DC / AC inverter circuit 103 and the C-phase bridge arm cross tubes Qc2 and Qc3 form a half-wave rectifier circuit. The two switch tubes Qa1 and Qa4 of the A-phase bridge arm are connected in series and then connected in parallel between the positive and negative DC busbars. One end of the circuit after the first protection resistor R3 and the first pre-charging switch K3 are connected in series is connected to the grid-connected relay KA2 of the A-phase, and the other end is connected to the midpoint of the A-phase bridge arm, that is, the connecting line between the two switch tubes Qa1 and Qa4. One end of the circuit after the second protection resistor R4 and the second pre-charging switch K4 are connected in series is connected to the grid-connected relay KC2 of the C-phase, and the other end is connected in series with the C-phase bridge arm cross tube, that is, the switch tubes Qc2 and Qc3, and then connected to the midpoint of the bus capacitor 102.

[0056] That is to say, the photovoltaic grid-connected inverter of this embodiment utilizes the freewheeling diode in the switch tube in the DC / AC inverter circuit 103 to construct a half-wave rectifier circuit. Specifically, when working, in the absence of DC input, the inverter issues a startup command, and the control unit 107 detects the bus capacitor voltage. If the bus capacitor voltage is less than the target voltage, the control unit 107 controls the absorption grid relays KA2 and KC2, and then absorbs the first pre-charge switch K3 and the second pre-charge switch K4 of the pre-charge circuit 108. The control unit 107 controls the switch tubes Qc2 and Qc3 of the C-phase bridge arm cross tube to be turned on, and the AC power grid 105 charges the positive and negative bus capacitors through the freewheeling diodes of the switch tubes Qa1 and Qa4 of the A-phase bridge arm and the switch tubes Qc2 and Qc3 of the C-phase bridge arm cross tube. When the voltages of C1 and C2 reach the target voltage, the first pre-charge switch K3 and the second pre-charge switch K4 of the pre-charge circuit 108 are disconnected. The control unit 107 can also disconnect KA2, KC2 and the first pre-charging switch K3 and the second pre-charging switch K4 when the bus capacitors C1 and C2 are over-voltage. The existence of the protection resistors R3 and R4 can further reduce the impact current when the AC grid 105 is connected to charge the bus capacitor 102.

[0057] Embodiment 7: This embodiment describes a method for starting the AC side of the photovoltaic grid-connected inverter as described in Embodiment 1. The input end of the pre-charging circuit 108 is directly connected to the AC grid 105. The working process of the method is as follows: Figure 8 As shown, it includes the following working steps: The startup state of the inverter is judged. If it is AC startup, the slow-start switch in the pre-charging circuit 108 is closed to charge the bus capacitor by connecting to the power grid. Otherwise, it is directly started and connected to the grid. The voltage of the bus capacitor 102 is detected. As the bus capacitor voltage gradually rises, if it is detected that the bus capacitor 102 reaches the voltage required for grid connection, the slow-start switch in the pre-charging circuit 108 is disconnected. After the disconnection is completed, the grid connection is started, otherwise the bus capacitor 102 is kept charged.

[0058] After disconnecting the slow-start switch in the pre-charging circuit 108, the insulation impedance of the inverter is also detected to prevent the DC input 101 side (PV side) from having too low impedance to the ground, and to prevent a large common-mode current from passing through the inverter after the inverter is running, causing damage to the device. If the insulation impedance to the ground obtained by the detection meets the grid-connected requirements, the control unit 107 detects again whether the bus capacitor voltage meets the voltage required for grid-connected. If it meets the voltage required for grid-connected, a control signal for controlling the on and off of each switch tube in the DC / AC inverter circuit 103 is generated based on the grid voltage sampling value to perform soft start of the grid-connected inverter; if the insulation impedance to the ground obtained by the detection does not meet the grid-connected requirements, a fault is reported; if the bus capacitor voltage does not meet the voltage required for grid-connected, the slow-start switch in the pre-charging circuit 108 is closed again, and the bus capacitor continues to be charged by connecting to the grid.

[0059] In the process of starting and stopping the pre-charging circuit, the current limiting resistor or the protection resistor is used to perform current limiting protection on the charging of the bus capacitor and to limit the common mode current impact.

[0060] The control unit 107 mainly calculates the duty cycle of the switch tube control signal through the grid voltage sampling value and the bus capacitor voltage value at this time, and obtains the pulse width modulation signal corresponding to the switch tube of each phase bridge arm, so that the DC / AC inverter circuit 103 outputs a sinusoidal voltage consistent with the amplitude and phase of the AC grid 105. Finally, when the grid-connected switch 104 is closed and connected to the AC grid 105, no impact current is generated, thereby realizing the AC soft start of the AC grid-connected inverter.

[0061] Embodiment 8: This embodiment describes a method for starting the AC side of the photovoltaic grid-connected inverter as described in Example 2. The input end of the pre-charging circuit 108 is connected to the midpoint of the grid-connected switch 104 (between the two grid-connected relays). The working process of the method is as follows: Fig. 9 As shown, it includes the following working steps: The startup state of the inverter is judged. If it is AC startup, the grid-connected relay connected to the pre-charging circuit 108 in the grid-connected switch 104 is closed, and the slow-start switch in the pre-charging circuit 108 is closed to charge the bus capacitor by connecting to the grid. Otherwise, the inverter is directly started and connected to the grid. The voltage of the bus capacitor 102 is detected. As the bus capacitor voltage gradually rises, if it is detected that the bus capacitor 102 reaches the voltage required for grid connection, the grid-connected relay connected to the pre-charging circuit 108 in the grid-connected switch 104 and the slow-start switch in the pre-charging circuit 108 are disconnected. After the disconnection is completed, the grid connection is started, otherwise the bus capacitor 102 is kept charged.

[0062] After disconnecting the grid-connected relay connected to the pre-charging circuit 108 in the grid-connected switch 104 and the slow-start switch in the pre-charging circuit 108, the insulation impedance of the inverter is also detected to prevent the DC input 101 side (PV side) from having too low impedance to the ground, and to prevent a large common-mode current from passing through the inverter after the inverter is running, causing damage to the device. If the insulation impedance to the ground obtained by the detection meets the grid-connected requirements, the control unit 107 detects again whether the bus capacitor voltage meets the voltage required for grid connection. If the voltage required for grid connection is met, a control signal for controlling the on and off of each switch tube in the DC / AC inverter circuit 103 is generated based on the grid voltage sampling value to perform soft start of the grid-connected inverter; if the insulation impedance to the ground obtained by the detection does not meet the grid-connected requirements, a fault is reported; if the bus capacitor voltage does not meet the voltage required for grid connection, the grid-connected relay connected to the pre-charging circuit 108 in the grid-connected switch 104 is closed again, and the slow-start switch in the pre-charging circuit 108 is closed, and the bus capacitor continues to be charged by connecting to the grid.

[0063] The control unit 107 mainly calculates the duty cycle of the switch tube control signal through the grid voltage sampling value and the bus capacitor voltage value at this time, and obtains the pulse width modulation signal corresponding to the switch tube of each phase bridge arm, so that the DC / AC inverter circuit 103 outputs a sinusoidal voltage consistent with the amplitude and phase of the AC grid 105. Finally, when the grid-connected switch 104 is closed, no impact current is generated, thereby realizing the AC soft start of the AC grid-connected inverter.

[0064] The above embodiments are only for illustrating the technical concept and features of the present invention, and their purpose is to enable people familiar with this technology to understand the contents of the present invention and implement them accordingly. They cannot be used to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. A photovoltaic grid-connected inverter, It is characterized in that It includes bus capacitor, DC / AC inverter circuit, control unit, pre-charge circuit and current limiting resistor; The external DC input is connected to the input end of the DC / AC inverter circuit via a DC bus, the bus capacitor is arranged between the DC input and the DC / AC inverter circuit, the bus capacitor is connected in parallel between the DC buses, and the output end of the DC / AC inverter circuit is connected to the power grid; The input end of the pre-charging circuit is connected to the current limiting resistor and then connected to the power grid, the output end of the pre-charging circuit is connected to the bus capacitor, and the pre-charging circuit is used to take power from the power grid to charge the bus capacitor; The bus capacitor is used to start the DC / AC inverter circuit after charging is completed; The control unit is used to control the operation of the pre-charging circuit, and control the DC / AC inverter circuit to output alternating current for grid connection after the bus capacitor is fully charged.

2. The photovoltaic grid-connected inverter according to claim 1, It is characterized in that The output end of the DC / AC inverter circuit is connected to the power grid via a grid-connected switch, the grid-connected switch includes at least one group of grid-connected relays, each group of grid-connected relays includes at least two relays connected in series, the pre-charging circuit is used to draw power from the grid-connected relay to charge the bus capacitor, and there is at least one relay between the power-drawing position and the DC / AC inverter circuit.

3. The photovoltaic grid-connected inverter according to claim 1, It is characterized in that The bus capacitor is divided into a positive bus capacitor and a negative bus capacitor, the midpoint of the bus capacitor is between the positive bus capacitor and the negative bus capacitor, the positive bus capacitor and the negative bus capacitor each have one capacitor or at least two capacitors, the at least two capacitors are connected in series or in parallel, and the positive bus capacitor and the negative bus capacitor are connected in series and then connected in parallel between the positive and negative DC buses of the inverter.

4. The photovoltaic grid-connected inverter according to claim 3, It is characterized in that The output end of the pre-charging circuit is divided into three connection points, which are respectively connected to the positive electrode, the negative electrode and the midpoint of the bus capacitor. The pre-charging circuit is used to charge the positive bus capacitor and the negative bus capacitor in the positive and negative half cycles of the power grid.

5. The photovoltaic grid-connected inverter according to claim 3, It is characterized in that The inverter is a single-phase inverter, a split-phase inverter or a three-phase inverter. The grid-connected switch includes at least two groups of grid-connected relays. A group of grid-connected relays is respectively connected to each phase AC bus of the inverter. The input end of the pre-charging circuit is connected to any two-phase output ends of the DC / AC inverter circuit via a current limiting resistor.

6. The photovoltaic grid-connected inverter according to any one of claims 1 to 5, It is characterized in that The pre-charging circuit includes a slow-start switch K1, a slow-start switch K2, a first diode D1 and a second diode D2, the slow-start switch K1 and the slow-start switch K2 are respectively connected in series with a current-limiting resistor to form a first branch and a second branch, one end of the first branch is connected to a phase of the power grid, and the other end is connected to the positive electrode of the first diode D1, the negative electrode of the first diode D1 is connected to the positive electrode of the bus capacitor, the positive electrode of the second diode D2 is connected to the negative electrode of the bus capacitor, one end of the second branch is connected to another phase of the power grid, and the other end is connected to the midpoint of the bus capacitor; The first diode D1 and the second diode D2 are used to charge the positive bus capacitor and the negative bus capacitor of the bus capacitor respectively in the positive and negative half cycles of the power grid.

7. The photovoltaic grid-connected inverter according to any one of claims 1 to 5, It is characterized in that The pre-charging circuit includes a slow-start switch K1, a first diode D1, a second diode D2, a protective resistor and a pre-charging switch. The slow-start switch K1 is connected in series with a current-limiting resistor to form a first branch. One end of the first branch is connected to a phase of a power grid, and the other end is connected to the positive electrode of the first diode D1. The negative electrode of the first diode D1 is connected to the positive electrode of the bus capacitor, and the positive electrode of the second diode D2 is connected to the negative electrode of the bus capacitor. The protective resistor and the pre-charging switch are connected in series to form a second branch. The second branch is connected in parallel to the two ends of another phase grid-connected relay.

8. The photovoltaic grid-connected inverter according to any one of claims 1 to 5, It is characterized in that The pre-charging circuit includes a slow-start switch K2, one end of the circuit formed by connecting the slow-start switch K2 and the current-limiting resistor in series is connected to one phase of the power grid, and the other end is connected to the midpoint of the bus capacitor to form a second circuit. The bridge arm in the DC / AC inverter circuit connected to the other phase of the power grid is used as the first circuit. The bridge arm includes two switching tubes connected in series. The bridge arm is connected in parallel with the bus capacitor and is used to charge the positive bus capacitor and the negative bus capacitor of the bus capacitor in the positive and negative half cycles of the power grid respectively.

9. The photovoltaic grid-connected inverter according to claim 8, It is characterized in that The first loop also includes a protection resistor and a pre-charge switch. The protection resistor and the pre-charge switch are connected in series to form a circuit which is connected in parallel to two ends of another phase grid-connected relay.

10. The photovoltaic grid-connected inverter according to claim 5, It is characterized in that The pre-charging circuit includes a first protection resistor, a first pre-charging switch, a second protection resistor and a second pre-charging switch. Any phase of the DC / AC inverter circuit includes a first bridge arm and a second bridge arm. The first bridge arm is two switch tubes connected in series in parallel to the bus capacitor, and the second bridge arm is two switch tubes connected in reverse series. One end of the second bridge arm is connected to the midpoint of the bus, and the other end is connected to the midpoint of the first bridge arm. One end of the first circuit formed by the first protection resistor and the first pre-charging switch being connected in series is connected to the grid-connected relay of one phase, and the other end is connected to the midpoint of the first bridge arm. One end of the second circuit formed by the second protection resistor and the second pre-charging switch being connected in series is connected to the grid-connected relay of another phase, and the other end is connected to the second bridge arm.

11. The photovoltaic grid-connected inverter according to claim 1, It is characterized in that It also includes an AC auxiliary power supply, the input end of which is connected to the power grid, and the output end of which is connected to the control unit, for supplying power to the control unit.

12. An AC side starting method of a photovoltaic grid-connected inverter according to any one of claims 1 to 11, It is characterized in that The following steps are included: The startup state of the inverter is judged. If it is AC startup, the slow-start switch in the pre-charging circuit is closed to charge the bus capacitor by connecting to the power grid. Otherwise, the inverter is directly started and connected to the grid. The voltage of the bus capacitor is detected. If the bus capacitor reaches the voltage required for grid connection, the slow-start switch in the pre-charging circuit is disconnected. After the disconnection is completed, the grid connection is started. Otherwise, the bus capacitor is kept charged. In the process of starting and stopping the pre-charging circuit, the current limiting resistor or the protection resistor is used to perform current limiting protection on the charging of the bus capacitor and to limit the common mode current impact.

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