Inverter grid-connected control circuit and control method

Abstract

The invention provides an inverter grid-connected control circuit and method, and the circuit comprises a conversion unit, a filtering unit, a relay unit, and a control unit, and the control unit is used for obtaining a starting instruction of an inverter; the capacitor voltage of the filtering unit is used as feed-forward voltage, and current loop starting is carried out; controlling the feed-forward voltage to gradually change to a power grid voltage; controlling the relay unit to be closed to complete grid connection; and controlling the feed-forward voltage to gradually change to the capacitor voltage. According to the invention, the feed-forward voltage is dynamically switched by stages, so that the impact current at the moment of starting and closing the relay is remarkably reduced, and the problem of overlarge impact current at the moment of closing the relay during grid connection of the inverter is solved.

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, the inverter grid-connected system, as the core equipment for the energy interaction between new energy devices such as photovoltaic power generation and energy storage systems and the power grid, has attracted extensive attention for its operating reliability and grid-connected quality. During the inverter grid-connected process, the transient characteristics of the system startup and the instantaneous closure of the grid-connected relay directly affect the equipment life and grid stability, especially the impact current problem has become a key factor restricting system performance.

[0003] In the inverter grid-connected system, when the system enters the grid-connected mode, if the inverter output is not precisely synchronized with the grid voltage and the grid-connected relay is closed, a significant impact current will be generated due to the difference in voltage amplitude, phase or frequency, which may damage the inverter switch tube, relay or other components, reduce system reliability, and seriously threaten system safety. In addition, under complex grid conditions (such as voltage distortion and frequency fluctuations), the dynamic response of the existing synchronization control algorithm is insufficient, which further exacerbates the generation of impact current. 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 instantaneous impact current when the relay is closed when the inverter is grid-connected.

[0005] The first aspect of an embodiment of the present application provides an inverter grid-connected control circuit, including 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 a power 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 a current loop; control the feedforward voltage to gradually change to the power 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.

[0006] In one of the embodiments, the control unit is also 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 of the embodiments, the control unit is further used 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 of the embodiments, 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; and the capacitor voltage is the voltage on the first filtering capacitor.

[0009] In one of the embodiments, the conversion unit includes a bridge arm, the input end of the filter 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 of the embodiments, the control unit is further used to: detect the state of the power grid after the relay unit is closed, and if the power grid fails, control the relay unit to open.

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

[0014] The second aspect of the embodiment of the present application provides an inverter grid-connected control method, which is applied to the above-mentioned inverter grid-connected control circuit, and 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; 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. The feedforward voltage is controlled to gradually change to the grid voltage, and the relay unit is controlled to close to complete the inverter grid connection, thereby reducing the voltage difference at both ends of the relay, significantly reducing the impact current, and 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 drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0017] Figure 1 A schematic diagram of the structure of an inverter grid-connected control circuit provided in one embodiment of the present application; Figure 2 A schematic diagram of the principle of an inverter grid-connected control circuit provided in one embodiment of the present application; Figure 3 A schematic diagram of the principle of an inverter grid-connected control circuit provided by another embodiment of the present application; Figure 4 A schematic diagram of a current loop control provided by an embodiment of the present application; Figure 5 A flow chart of an inverter grid-connected control method provided in one embodiment of the present application. DETAILED DESCRIPTION

[0018] In order to make the purpose, technical solution and advantages of the present application more clearly understood, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0019] It should be noted that when an element is referred to as being "fixed to" or "disposed on" another element, it can 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 can be directly connected to the other element or indirectly connected to the other element.

[0020] It should be understood that the orientation or position relationship indicated by terms such as "length", "width", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside" and "outside" are based on the orientation or position relationship shown in the drawings, and are only for the convenience of describing the present 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 should not be understood as a limitation on the present application.

[0021] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of this application, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0022] 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, and the DC voltage is, for example, 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, and is used to merge the output voltage into the power grid when the relay unit 300 is closed, so as to realize the 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 the capacitor voltage on the filter unit 200 is used as a feed-forward voltage to start the current loop. The feed-forward voltage is controlled to gradually change to the grid voltage, and the relay unit 300 is controlled to close to complete the grid connection, and finally the feed-forward voltage is controlled to gradually change to the capacitor voltage on the filter unit 200.

[0023] In the 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, the voltage on the DC bus of the inverter side is reduced. The impact of the filter unit 200 on the voltage is reduced, and the feedforward voltage is gradually changed to the grid voltage by controlling the relay unit 300 to close to complete the grid connection of the inverter, reduce the voltage difference across the relay, and significantly reduce the impact 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, and significantly reduces the current impact at the moment of grid connection.

[0024] In one embodiment, see Figure 1The control unit 400 is also used to send a closing control signal to the relay unit 300, detect the state of the relay unit 300, and if the relay unit 300 is in a closed state, control the feedforward voltage to gradually change to the capacitor voltage. If the relay unit 300 is in an open state, maintain the feedforward voltage and send a closing signal to the relay unit 300 again. By monitoring the state of the relay unit 300, voltage mismatch caused by relay malfunction (such as failure to attract or accidental disconnection) is avoided, fault tolerance is improved, misjudgment caused by transient interference (such as power grid fluctuations or signal delays) is avoided, the number of invalid operations is reduced, and the life of the relay is extended.

[0025] 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 and reduce 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.

[0026] In one embodiment, see Figure 1 and Figure 2 , the filter unit 200 is an LCL filter, the filter unit 200 includes a first filter inductor L1, a second filter inductor L2 and a first filter capacitor C1, and the capacitor voltage is the voltage on the first filter capacitor C1. It can be understood that the inverter provided in the embodiment of the present application is a three-phase inverter, and accordingly, the filter unit 200 has three LCL filters, which correspond to the three phases of the inverter respectively.

[0027] 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 conversion circuit topology of a three-phase inverter, including a bridge arm circuit composed of six switch tubes. For each phase, the output end of the three-phase bridge arm, that is, 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 switch tubes Q1, Q2, Q3, Q4, Q5, and Q6 by sending a PWM control wave to realize the conversion from DC to AC.

[0028] 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 composed of the switch tubes 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, and 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 can filter out the harmonics caused by the high-frequency action of the switch tube (such as the PWM carrier frequency), the first filter capacitor C1 is used to absorb the high-frequency ripple current, and the second filter inductor L2 further attenuates the residual high-frequency component. The voltage output at the midpoint of the bridge arm is buffered by the first filter inductor L1 to avoid the sudden change of the DC bus voltage 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 between the two ends of the relay at the moment of closing.

[0029] 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, and the second relay K2 is connected to the grid. The control unit 400 is also used to control the feedforward voltage to gradually change to the grid voltage. If the current of the first filter inductor L1 is zero, the first relay K1 is controlled to close for a period of time, and then the second relay K2 is controlled to close. When the inductor current on the first filter inductor L1 is zero, the first relay K1 is closed, and the second relay K2 is closed through the delayed closing of the second relay K2. After the DC bus voltage further matches the grid voltage, the second relay K2 is closed to prevent the second relay K2 from directly bearing the high voltage difference impact.

[0030] 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.

[0031] In one embodiment, see Figure 1 , Figure 2 and Figure 3, the 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, at which time the filter unit 200 forms an LC filter to reduce the initial charging current. During the grid-connected phase, the third relay K3 is closed, at which time the second filter inductor L2 is introduced to form an LCL filter to enhance the high-frequency harmonic filtering capability. If a short circuit / open circuit fault occurs in the first filter inductor L1 or the second filter inductor L2, the control unit 400 can quickly disconnect the third relay K3 to isolate the fault branch, thereby improving the reliability of the inverter grid-connected control circuit.

[0032] In one embodiment, see Figure 1 and Figure 2 The control unit 400 is also used to detect the state of the power grid after the relay unit 300 is closed. If the power grid fails, the relay unit 300 is controlled to disconnect. By real-time detection of parameters such as power grid voltage, frequency, and phase, once a power grid abnormality (such as overvoltage, undervoltage, frequency offset, short circuit, etc.) is identified, the relay unit 300 is controlled to disconnect and isolate the faulty power grid to prevent the fault from affecting the DC-side photovoltaic array or energy storage battery, thereby protecting the safety of upstream equipment.

[0033] In one embodiment, see Figure 1 and Figure 2 The control unit 400 is also used to output a PMW signal to drive the switch tube of the conversion unit 100.

[0034] Further, in one embodiment, see Figure 4 , Figure 4 The schematic diagram of the current loop control provided in the embodiment of the present application is shown. The current is given minus the inductor current feedback, and the current passes through the current loop controller and the feedforward voltage is added. 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.

[0035] In the 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, the voltage on the DC bus of the inverter side is reduced. The impact of the filter unit 200 on the voltage is reduced, and the feedforward voltage is gradually changed to the grid voltage by controlling the relay unit 300 to close to complete the grid connection of the inverter, reduce the voltage difference across the relay, and significantly reduce the impact 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, and significantly reduces the current impact at the moment of grid connection.

[0036] 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 comprises the steps of: S100: Obtain a startup instruction for the inverter.

[0037] S200 , using the capacitor voltage of the filter unit 200 as a feed-forward voltage to start the current loop.

[0038] 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.

[0039] S300, controlling the feedforward voltage to gradually change to the grid voltage.

[0040] S400 , controlling the relay unit 300 to close to complete grid connection.

[0041] S500, controlling the feedforward voltage to gradually change to the capacitor voltage.

[0042] The inverter control method provided in the embodiment of the present application significantly reduces the impact current at the moment of starting and relay closure by dynamically switching the feedforward voltage in stages and combining the current loop coordinated control. This solution is compatible with a variety of new energy application scenarios, has high reliability and scalability, and can effectively extend the life of the equipment and improve the quality of power.

[0043] The embodiments described above 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, a person skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. Such modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present application, and should all be included in the protection scope of the present application.

Claims

1. An inverter grid-connected control circuit, characterized in that: The invention comprises 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, the relay unit is connected to the filtering unit, and is used to merge the output voltage into a power 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: Get the startup command of the inverter; Using the capacitor voltage of the filter unit as a feed-forward voltage to start the current loop; Controlling the feedforward voltage to gradually change to a 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.

2. The inverter grid-connected control circuit according to claim 1, characterized in that: The control unit is also used for: sending a closing control signal to the relay unit; The state of the relay unit is detected. If the relay unit is in a closed state, the feed-forward voltage is controlled to gradually change to the capacitor voltage. If the relay unit is in an open state, the feed-forward voltage is maintained and a closing signal is sent to the relay unit again.

3. The inverter grid-connected control circuit according to claim 1, characterized in that: The control unit is also used for: The inductor current of the filter unit is obtained, and when the inductor current is less than a preset threshold, a closing control signal is sent to the relay unit.

4. The inverter grid-connected control circuit according to claim 1, characterized in that: 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.

5. The inverter grid-connected control circuit according to claim 4, characterized in that: The conversion unit comprises a bridge arm, the input end of the filter 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.

6. The inverter grid-connected control circuit according to claim 5, characterized in that: 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.

7. The inverter grid-connected control circuit according to claim 6, characterized in that: The relay unit comprises 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 control unit is further used for: After the feed-forward 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.

8. The inverter grid-connected control circuit according to claim 1, characterized in that: The control unit is also used for: After the relay unit is closed, the state of the power grid is detected, and if the power grid fails, the relay unit is controlled to be opened.

9. The inverter grid-connected control circuit according to claim 1, characterized in that: The control unit is also used for: The PMW signal is output to drive the switch tube of the conversion unit.

10. An inverter grid-connected control method, applied to the inverter grid-connected control circuit according to any one of claims 1 to 9, characterized in that: include: Get the startup command of the inverter; Using the capacitor voltage of the filter unit as a feed-forward voltage to start the current loop; Controlling the feedforward voltage to gradually change to a 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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