Inverter grid-connected control circuit and control method

Abstract

The present application provides an inverter grid-connected control circuit and control method, including a conversion unit, a filter unit, a relay unit, and a control unit, wherein the control unit is used to: obtain the inverter startup instruction; use the capacitor voltage of the filter unit as the feedforward voltage to start the current loop; control the feedforward voltage to gradually change to the grid voltage; control the relay unit to close to complete the grid connection; and control the feedforward voltage to gradually change to the capacitor voltage. The present application significantly reduces the inrush current at the moment of startup and relay closure by dynamically switching the feedforward voltage in stages, thereby solving the problem of excessive inrush current at the moment of relay closure when the inverter is grid-connected.

Description

Technical Field

[0001] The present application relates to the field of power electronics technology, and in particular to an inverter grid-connected control circuit and a control method. Background Art

[0002] With the rapid development of renewable energy generation technology, grid-connected inverter systems, as the core equipment for energy interaction between new energy devices such as photovoltaic power generation and energy storage systems and the power grid, have attracted widespread attention for their operational reliability and grid connection quality. During the inverter grid connection process, the transient characteristics of the system startup and the moment the grid-connection relay is activated directly affect the equipment lifespan and grid stability. In particular, the inrush current problem has become a key factor restricting system performance.

[0003] In an inverter-connected grid-connected system, if the inverter output is not precisely synchronized with the grid voltage when the grid-connection relay is closed during grid-connection mode, differences in voltage amplitude, phase, or frequency will generate significant inrush currents. This can damage the inverter switches, relays, or other components, reducing system reliability and posing a serious threat to safety. Furthermore, existing synchronization control algorithms lack the dynamic response required under complex grid conditions (such as voltage distortion and frequency fluctuations), further exacerbating the generation of inrush currents. Summary of the Invention

[0004] In view of this, an embodiment of the present application provides an inverter grid-connected control circuit and a control method, which aim to solve the technical problem of excessive inrush current when the relay is closed when the inverter is grid-connected.

[0005] A first aspect of an embodiment of the present application provides an inverter grid-connected control circuit, comprising a conversion unit, a filtering unit, a relay unit, and a control unit, wherein the conversion unit is used to convert a DC voltage into an AC output voltage, the filtering unit is used to filter the output voltage, and the relay unit is connected to the filtering unit for connecting the output voltage to the grid when the relay unit is closed; the control unit is connected to the conversion unit, the filtering unit, and the relay unit; the control unit is used to: obtain a start-up instruction for the inverter; use the capacitor voltage of the filtering unit as a feedforward voltage to start the current loop; control the feedforward voltage to gradually change to the grid voltage; control the relay unit to close to complete grid connection; and control the feedforward voltage to gradually change to the capacitor voltage.

[0006] In one embodiment, the control unit is further used to: send a closing control signal to the relay unit; detect the state of the relay unit, and if the relay unit is in a closed state, control the feedforward voltage to gradually change to the capacitor voltage; if the relay unit is in an open state, maintain the feedforward voltage and send a closing signal to the relay unit again.

[0007] In one embodiment, the control unit is further configured to: obtain the inductor current of the filter unit, and send a closing control signal to the relay unit when the inductor current is less than a preset threshold.

[0008] In one embodiment, the filtering unit is an LCL filter, and the filtering unit includes a first filtering inductor, a second filtering inductor, and a first filtering capacitor; the capacitor voltage is the voltage on the first filtering capacitor.

[0009] In one embodiment, the conversion unit includes a bridge arm, the input end of the filtering unit is connected to the output end of the bridge arm, and the output end of the LCL filter is connected to the relay unit.

[0010] In one embodiment, one end of the first filter inductor is connected to the midpoint of the bridge arm, the other end of the first filter inductor is connected to the first end of the first filter capacitor and the first end of the second filter inductor, and the other end of the second filter inductor is connected to the relay unit.

[0011] In one embodiment, the relay unit includes a first relay and a second relay, and the first relay and the second relay are connected in sequence between the filter unit and the power grid; the control unit is also used to: after controlling the feedforward voltage to gradually change to the power grid voltage, if the current of the first filter inductor is zero, control the first relay to close for a period of time, and then control the second relay to close.

[0012] In one embodiment, the control unit is further configured to: detect the state of the power grid after the relay unit is closed, and control the relay unit to be disconnected if the power grid fails.

[0013] In one embodiment, the control unit is further configured to output a PMW signal to drive a switch tube of the conversion unit.

[0014] A second aspect of an embodiment of the present application provides an inverter grid-connected control method, which is applied to the above-mentioned inverter grid-connected control circuit. The control method includes: obtaining a start-up instruction of the inverter; using the capacitor voltage of the filter unit as the feedforward voltage to start the current loop; controlling the feedforward voltage to gradually change to the grid voltage; controlling the relay unit to close to complete the grid connection; and controlling the feedforward voltage to gradually change to the capacitor voltage.

[0015] The beneficial effects of the embodiments of the present application are as follows: the inverter grid-connected control circuit includes a conversion unit, a filter unit, a relay unit and a control unit. When the inverter is connected to the grid, the capacitor voltage of the filter unit is used as the feedforward voltage to start the current loop, thereby reducing the current impact of the DC bus on the filter unit. By controlling the feedforward voltage to gradually change to the grid voltage, the relay unit is controlled to close to complete the grid connection of the inverter, reducing the voltage difference across the relay and significantly reducing the impact current. After the relay unit is closed, the feedforward voltage is controlled to gradually change to the capacitor voltage. By dynamically adjusting the feedforward voltage in stages and combining the current loop control strategy, the stability and safety of the inverter grid connection are improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for use in the embodiments or descriptions of the prior art. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0017] Figure 1 A schematic diagram of the structure of an inverter grid-connected control circuit provided in one embodiment of the present application;

[0018] Figure 2 A schematic diagram of the principle of an inverter grid-connected control circuit provided in one embodiment of the present application;

[0019] Figure 3 A schematic diagram of the principle of an inverter grid-connected control circuit provided by another embodiment of the present application;

[0020] Figure 4 A schematic diagram of a current loop control system according to an embodiment of the present application;

[0021] Figure 5 This is a flow chart of an inverter grid-connected control method provided in one embodiment of the present application. DETAILED DESCRIPTION

[0022] In order to make the purpose, technical solutions and advantages of this application more clearly understood, the present application is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain this application and are not intended to limit this application.

[0023] It should be noted that when an element is referred to as being “fixed on” or “disposed on” another element, it may be directly on the other element or indirectly on the other element. When an element is referred to as being “connected to” another element, it may be directly connected to the other element or indirectly connected to the other element.

[0024] It should be understood that the terms "length", "width", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the accompanying drawings, and are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on this application.

[0025] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of the technical features being referred to. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the features. Throughout the description of this application, "plurality" means two or more, unless otherwise specifically defined.

[0026] See also Figure 1 As shown, an embodiment of the present application provides an inverter grid-connected control circuit, including a conversion unit 100, a filter unit 200, a relay unit 300 and a control unit 400. The conversion unit 100 is used to convert a DC voltage into an AC output voltage, such as a DC power supply of an energy storage battery, a DC power supply provided by a photovoltaic panel, etc. The filter unit 200 is used to filter the output voltage of the conversion unit, and the relay unit 300 is connected to the filter unit 200 for connecting the output voltage to the power grid when the relay unit 300 is closed, thereby realizing grid-connected switching of the inverter. The control unit 400 is connected to the conversion unit 100, the filter unit 200 and the relay unit 300. Among them, the control unit 400 is used to obtain the start-up instruction of the inverter, and uses the capacitor voltage on the filter unit 200 as a feed-forward voltage to start the current loop. The feed-forward voltage is controlled to gradually change to the grid voltage, the relay unit 300 is controlled to close to complete grid connection, and finally the feed-forward voltage is controlled to gradually change to the capacitor voltage on the filter unit 200.

[0027] In an embodiment of the present application, when the inverter is connected to the grid, the capacitor voltage on the filter unit 200 is used as the feedforward voltage to start the current loop, thereby reducing the impact of the voltage on the DC bus of the inverter on the filter unit 200, and gradually changing the feedforward voltage to the grid voltage by controlling the relay unit 300 to close to complete the grid connection of the inverter, thereby reducing the voltage difference across the relay and significantly reducing the inrush current. After the relay unit 300 is closed, the feedforward voltage is gradually changed to the capacitor voltage. The embodiment of the present application improves the stability and safety of the inverter grid connection by dynamically adjusting the feedforward voltage in stages and combining the current loop control strategy, thereby significantly reducing the current impact at the moment of grid connection.

[0028] In one embodiment, see Figure 1The control unit 400 is further configured to send a closing control signal to the relay unit 300 and detect the state of the relay unit 300. If the relay unit 300 is closed, the feedforward voltage is gradually changed to the capacitor voltage. If the relay unit 300 is open, the feedforward voltage is maintained and a closing signal is sent to the relay unit 300 again. By monitoring the state of the relay unit 300, voltage mismatches caused by relay malfunctions (such as failed closure or accidental disconnection) are avoided, fault tolerance is improved, misjudgments caused by transient interference (such as power grid fluctuations or signal delays) are avoided, the number of invalid operations is reduced, and the relay life is extended.

[0029] In one embodiment, see Figure 1 The control unit 400 is also used to obtain the inductor current of the filter unit 200, and send a closing control signal to the relay unit 300 when the inductor current is less than a preset threshold. The inductor current of the filter unit 200 directly reflects the energy exchange intensity between the inverter and the power grid. When the current is lower than the preset threshold, it indicates that the energy flow of the system tends to be stable. At this time, closing the relay can greatly reduce the impact caused by the sudden change of current. In one embodiment, the preset threshold is zero, that is, when the inductor current tends to zero, it is considered that the feedforward voltage has completely gradually changed to the grid voltage. At this time, the relay unit 300 is controlled to close. Through real-time current feedback, the control unit 400 can quickly determine the best closing time, reducing the energy loss caused by relying on fixed delays in traditional solutions (such as repeated charging and discharging of bus capacitors). In some embodiments, the preset threshold can be greater than zero.

[0030] In one embodiment, see Figure 1 and Figure 2 Filter unit 200 is an LCL filter, comprising a first filter inductor L1, a second filter inductor L2, and a first filter capacitor C1. The capacitor voltage is the voltage across first filter capacitor C1. It is understood that the inverter provided in the embodiment of the present application is a three-phase inverter. Accordingly, filter unit 200 includes three LCL filters, one corresponding to each of the three phases of the inverter.

[0031] In one embodiment, see Figure 1 and Figure 2 The conversion unit 100 includes a bridge arm. The input end of the filter unit 200 is connected to the output end of the bridge arm, and the output end of the LCL filter is connected to the relay unit 300. It can be understood that the conversion unit 100 is a three-phase inverter conversion circuit topology, including a bridge arm circuit composed of six switching tubes. For each phase, the output end of the three-phase bridge arm, i.e., the midpoint of the bridge arm, is respectively connected to the input end of the corresponding LCL filter. The control unit 400 controls the on and off of the switching tubes Q1, Q2, Q3, Q4, Q5, and Q6 by sending PWM control waves, achieving DC to AC conversion.

[0032] In one embodiment, see Figure 1 and Figure 2 Taking one of the three phases as an example, one end of the first filter inductor L1 is connected to the midpoint of the bridge arm formed by the switching transistors Q1 and Q2. The other end of the first filter inductor L1 is connected to the first end of the first filter capacitor C1 and the first end of the second filter inductor L2. The other end of the second filter inductor L2 is connected to the relay unit 300. For the LCL filter, the first filter inductor L1 filters out harmonics caused by the high-frequency operation of the switching transistors (such as the PWM carrier frequency). The first filter capacitor C1 absorbs high-frequency ripple current, and the second filter inductor L2 further attenuates residual high-frequency components. The voltage output at the bridge arm midpoint is buffered by the first filter inductor L1 to prevent sudden changes in the DC bus voltage from being directly transmitted to the first filter capacitor C1. The relay unit 300 is connected to the rear end of the second filter inductor L2. When the relay unit 300 is closed, the LCL filter has established a steady-state voltage, reducing the voltage difference across the relay at the moment of closing.

[0033] In one embodiment, see Figure 1 、 Figure 2 and Figure 3 The relay unit 300 includes a first relay K1 and a second relay K2, which are sequentially connected between the filter unit 200 and the grid, with the second relay K2 connected to the grid. The control unit 400 is further configured to control the feedforward voltage to gradually change to the grid voltage. If the current in the first filter inductor L1 is zero, the first relay K1 is closed for a period of time before the second relay K2 is closed. When the inductor current in the first filter inductor L1 is zero, the first relay K1 is closed, and the second relay K2 is closed after the DC bus voltage further matches the grid voltage, preventing the second relay K2 from being directly subjected to the high voltage difference.

[0034] In the embodiment of the present application, during the closing stage of the first relay K1, the feed-forward voltage has gradually changed to the grid voltage. At this time, the closing of the first relay K1 only needs to balance the residual voltage difference (such as the slight difference between the inductance of the second filter inductor L2 and the capacitive load on the grid side). The delayed closing of the second relay K2 provides additional buffer time for grid connection, ensuring that the bus capacitor is fully charged and the inductor current is completely stable.

[0035] In one embodiment, see Figure 1 、 Figure 2 and Figure 3The relay unit 300 also includes a third relay K3, which is connected between the first filter inductor L1 and the second filter inductor L2. The third relay K3 is disconnected during the inverter startup phase, during which the filter unit 200 forms an LC filter, reducing the initial charging current. During the grid-connected phase, the third relay K3 is closed, introducing the second filter inductor L2 to form an LCL filter, enhancing high-frequency harmonic filtering capabilities. If a short circuit / open circuit fault occurs in the first filter inductor L1 or the second filter inductor L2, the control unit 400 quickly disconnects the third relay K3 to isolate the faulty branch, improving the reliability of the inverter's grid-connected control circuit.

[0036] In one embodiment, see Figure 1 and Figure 2 The control unit 400 is also used to monitor the grid status after the relay unit 300 is closed. If a grid fault occurs, the control unit 400 controls the relay unit 300 to disconnect. By monitoring grid voltage, frequency, phase, and other parameters in real time, if a grid anomaly (such as overvoltage, undervoltage, frequency offset, or short circuit) is detected, the control unit 400 controls the relay unit 300 to disconnect, isolating the faulty grid and preventing the fault from affecting the DC-side photovoltaic array or energy storage battery, thereby protecting upstream equipment.

[0037] In one embodiment, see Figure 1 and Figure 2 The control unit 400 is further configured to output a PMW signal to drive the switch tube of the conversion unit 100 .

[0038] Further, in one embodiment, see Figure 4 , Figure 4 The diagram shown is a schematic diagram of the current loop control provided in an embodiment of the present application. The current is given minus the inductor current feedback, passes through the current loop controller, and is added with the feedforward voltage. The obtained value is compared with the carrier to generate a PWM control signal, which passes through the inverter hardware topology structure, namely the conversion unit 100, to generate voltage and current.

[0039] In an embodiment of the present application, when the inverter is connected to the grid, the capacitor voltage on the filter unit 200 is used as the feedforward voltage to start the current loop, thereby reducing the impact of the voltage on the DC bus of the inverter on the filter unit 200, and gradually changing the feedforward voltage to the grid voltage by controlling the relay unit 300 to close to complete the grid connection of the inverter, thereby reducing the voltage difference across the relay and significantly reducing the inrush current. After the relay unit 300 is closed, the feedforward voltage is gradually changed to the capacitor voltage. The embodiment of the present application improves the stability and safety of the inverter grid connection by dynamically adjusting the feedforward voltage in stages and combining the current loop control strategy, thereby significantly reducing the current impact at the moment of grid connection.

[0040] The present application also provides an inverter grid-connected control method, which is applied to the above-mentioned inverter grid-connected control circuit. Figure 5 , the inverter grid-connected control method includes the following steps:

[0041] S100: Obtain a startup instruction for the inverter.

[0042] S200 , using the capacitor voltage of the filter unit 200 as a feedforward voltage to start the current loop.

[0043] Please combine Figure 2 As shown, in this embodiment, the capacitor voltage of the filter unit 200 is the voltage on the first filter capacitor C1.

[0044] S300 , controlling the feedforward voltage to gradually change to the grid voltage.

[0045] S400 , controlling the relay unit 300 to close to complete grid connection.

[0046] S500 , controlling the feedforward voltage to gradually change to the capacitor voltage.

[0047] An inverter control method provided in this application significantly reduces the inrush current during startup and relay closure by dynamically switching the feedforward voltage in stages and combining it with current loop coordinated control. This solution is compatible with a variety of new energy application scenarios, offers high reliability and scalability, and can effectively extend equipment life and improve power quality.

[0048] The above-described embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present application, and should all be included in the scope of protection of the present application.

Claims

1. An inverter grid-connected control circuit, characterized in that: The system comprises a conversion unit, a filtering unit, a relay unit and a control unit. The conversion unit is used to convert a DC voltage into an AC output voltage. The filtering unit is used to filter the output voltage. The relay unit is connected to the filtering unit and is used to incorporate the output voltage into the power grid when the relay unit is closed. The control unit is connected to the conversion unit, the filtering unit and the relay unit respectively. The control unit is used to: Get the inverter startup command; Using the capacitor voltage of the filter unit as a feedforward voltage to start the current loop; Controlling the feedforward voltage to gradually change to the grid voltage; Controlling the relay unit to close to complete grid connection; Controlling the feedforward voltage to gradually change to the capacitor voltage; The filtering unit is an LCL filter, and the filtering unit includes a first filter inductor, a second filter inductor and a first filter capacitor; the capacitor voltage is the voltage on the first filter capacitor; The control of the current loop includes: subtracting the inductor current feedback of the first filter inductor from the given current, adding the feedforward voltage through the current loop controller, comparing the obtained value with the carrier, and generating a PWM control signal; The relay unit includes a first relay and a second relay, wherein the first relay and the second relay are sequentially connected between the filter unit and the power grid; The inverter is a three-phase inverter, and the conversion unit is a conversion circuit topology of the three-phase inverter; the conversion unit includes a bridge arm, the input end of the LCL filter is connected to the output end of the bridge arm, and the output end of the LCL filter is connected to the first relay; One end of the first filter inductor is connected to the midpoint of the bridge arm, the other end of the first filter inductor is connected to the first end of the first filter capacitor and the first end of the second filter inductor, and the other end of the second filter inductor is connected to the first relay; The relay unit further includes a third relay connected between the first filter inductor and the second filter inductor; The control unit is further configured to control the third relay to be disconnected during the startup phase of the inverter, and control the third relay to be closed during the grid-connected phase of the inverter.

2. The inverter grid-connected control circuit according to claim 1, characterized in that: The control unit is further configured to: After the feedforward voltage is controlled to gradually change to the grid voltage, if the current of the first filter inductor is zero, the first relay is controlled to be closed for a period of time, and then the second relay is controlled to be closed.

3. The inverter grid-connected control circuit according to claim 1, characterized in that: The control unit is further configured to: After the first relay, the second relay, and the third relay of the relay unit are all closed, the grid state is detected. If the grid fails, the first relay, the second relay, and the third relay of the relay unit are controlled to be open.

4. The inverter grid-connected control circuit according to claim 1, wherein: The control unit is further configured to: Outputting a PMW signal to drive the switch tube of the conversion unit.

5. An inverter grid-connected control method, applied to the inverter grid-connected control circuit according to any one of claims 1 to 4, characterized in that: include: Get the inverter startup command; Using the capacitor voltage of the filter unit as a feedforward voltage to start the current loop; Controlling the feedforward voltage to gradually change to the grid voltage; Controlling the relay unit to close to complete grid connection; The feed-forward voltage is controlled to gradually change to the capacitor voltage.

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