Inverter and method for starting inverter

Abstract

The invention relates to the technical field of grid-connected inverters, and discloses an inverter and an inverter starting method, and the method comprises the following steps: issuing a starting instruction to the inverter, and closing a first AC relay; measuring inverter grid-connected point voltage, filter capacitor voltage and filter inductor current, and further processing to obtain an input signal of a current controller; the inverter circuit is driven through an output signal of the current controller, so that the voltage of the filter capacitor enters a voltage following state; and determining whether the inverter enters a normal working mode or not according to a preset second alternating-current relay closing judgment logic. The problem that the inrush current is too large when the grid-connected inverter is started is solved, the risk that the alternating-current relay is burnt out is avoided, and therefore the reliability of the grid-connected inverter connected with a power grid is guaranteed, and high engineering practicability is achieved.

Description

[0001] This application is a divisional application. The application number of the original application is 202211546956.2, and the application date is December 5, 2022. The entire contents of the original application are incorporated into this application by reference. Technical Field

[0002] The present invention relates to the technical field of grid-connected inverters, and in particular to an inverter and a method for starting the inverter. Background Art

[0003] Voltage source grid-connected inverters using pulse width modulation technology play an indispensable role in the fields of photovoltaic wind power and other new energy power generation, power quality management, etc. They can convert the DC power stored on the bus capacitor into AC power synchronized with the mains, solving problems such as power shortage or power quality in the grid. However, when the grid-connected inverter is connected to the grid, there is a situation where the voltage difference between the grid and the inverter output filter capacitor is too large. When the AC relay is closed, a large inrush current will be generated, causing the AC relay on the inverter output side to stick, and it will also pose a certain threat to the power safety of users near the inverter grid connection point. Summary of the invention

[0004] In view of the deficiencies and defects in the prior art, the present invention provides an inverter and an inverter starting method, which solves the problem of excessive inrush current when the grid-connected inverter is started, avoids the risk of burning out the AC relay, thereby ensuring the reliability of the grid-connected inverter connecting to the power grid, and does not require additional hardware protection devices.

[0005] The purpose of the present invention can be achieved by the following technical solutions:

[0006] An inverter and a method for starting the inverter. The inverter comprises a DC bus capacitor, a DC bus capacitor precharging circuit, a first AC relay, a DC / AC inverter circuit including a current controller module, a filter capacitor, a filter inductor, and a second AC relay.

[0007] The connection relationship is as follows: the DC bus capacitor pre-charging circuit, the DC bus capacitor, and the DC / AC inverter circuit are connected in parallel in sequence; one end of the filter inductor is connected to the DC / AC inverter circuit, and the other end is connected to one end of the filter capacitor and one end of the second AC relay; the other end of the filter capacitor is connected to the DC / AC inverter circuit and is connected in parallel to one end of the AC power grid as an inverter; one end of the first AC relay is connected to the DC bus capacitor pre-charging circuit, and the other end is connected to the other end of the second AC relay and is connected in parallel to the other end of the AC power grid as an inverter.

[0008] The method comprises the following steps:

[0009] S1: Send a start-up command to the inverter, and the first AC relay is closed.

[0010] S2: Measure the inverter grid-connected point voltage, filter capacitor voltage, and filter inductor current, and further process them to obtain the input signal of the current controller.

[0011] S3: The output signal of the current controller drives the inverter circuit so that the filter capacitor voltage enters the voltage following state, that is, the filter capacitor voltage phase is synchronized with the phase of the fundamental component of the grid-connected point voltage, and the filter capacitor voltage amplitude slowly rises from 0 and remains within the range of a% to b% of the amplitude of the fundamental component of the grid-connected point voltage.

[0012] S4: Determine whether the inverter enters the normal working mode according to the preset second AC relay closing judgment logic, specifically:

[0013] S41, calculate the filter capacitor voltage u C and the filter capacitor voltage reference value u cref The difference Err = u C -u cref ;

[0014] S42, if u cref If it is within the preset range, jump to the next step, otherwise jump to S2;

[0015] S43, if Err is within the preset range, jump to the next step, otherwise jump to S2;

[0016] S44, if u cref / (u amp cosθ) is greater than the preset threshold, jump to the next step, otherwise jump to S2; where u amp is the amplitude of the fundamental component of the voltage at the inverter grid connection point, θ is the phase of the fundamental component of the voltage at the inverter grid connection point;

[0017] S45, if u C If the amplitude reaches the preset threshold value for a period of time within one power frequency cycle, the second AC relay is closed, the first AC relay is disconnected, and the inverter enters the normal working mode; otherwise, it jumps to S2.

[0018] Furthermore, in S3, a=75, b=90.

[0019] Furthermore, the current controller is a hysteresis controller.

[0020] Furthermore, the second AC relay is in an open state before being closed in step S4.

[0021] The beneficial technical effects of the present invention are as follows: it solves the problem of excessive inrush current of the grid-connected inverter at startup, avoids the risk of burning out the AC relay, thereby ensuring the reliability of the grid-connected inverter and the power safety of users, and does not require additional hardware protection devices, thereby reducing product costs. In addition, there is no need to additionally design a current controller, thereby reducing development costs. The voltage control accuracy of the filter capacitor in the voltage following state is not high, and the relay logic judgment conditions are not complicated, which greatly improves the reliability and portability of the algorithm and has strong engineering practicality. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is the electrical diagram of the inverter in the present invention.

[0023] Figure 2 This is a control structure diagram of the inverter in the present invention.

[0024] Figure 3 This is a closing judgment logic diagram of the second AC relay in an embodiment of the present invention.

[0025] Figure 4 It is the overall flow chart of the present invention.

[0026] Figure numbers: 1 is the AC power grid, 2 is the transmission line impedance, 3 is the user load, and 4 is the inverter device; among them, 41 is the DC bus capacitor pre-charging circuit, 42 is the DC bus capacitor, 43 is the filter inductor, 44 is the filter capacitor, 45 is the DC / AC inverter circuit, s1 is the first AC relay, and s2 is the second AC relay. DETAILED DESCRIPTION

[0027] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention 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 invention and do not limit the present invention.

[0028] Example:

[0029] In the embodiment, there are a transmission line impedance 2 and a user load 3 in the AC power grid 1 .

[0030] like Figure 1 , 4 As shown, an inverter and a method for starting the inverter are shown, wherein the inverter 4 includes a DC bus capacitor 42, a DC bus capacitor pre-charging circuit 41, a first AC relay s1, a DC / AC inverter circuit 45 including a hysteresis controller module, a filter capacitor 44, a filter inductor 43, and a second AC relay s2.

[0031] The connection relationship is as follows: the DC bus capacitor pre-charging circuit 41, the DC bus capacitor 42, and the DC / AC inverter circuit 45 are connected in parallel in sequence; one end of the filter inductor 43 is connected to the DC / AC inverter circuit 45, and the other end is connected to one end of the filter capacitor 44 and one end of the second AC relay s2; the other end of the filter capacitor 44 is connected to the DC / AC inverter circuit 45 and is connected in parallel to one end of the AC power grid 1 as an inverter 4; one end of the first AC relay s1 is connected to the DC bus capacitor pre-charging circuit 41, and the other end is connected to the other end of the second AC relay s2 and is connected in parallel to the other end of the AC power grid 1 as an inverter 4.

[0032] The method comprises the following steps:

[0033] S1: a start-up command is issued to the inverter 4, the first AC relay s1 is closed, and the phase voltage charges the DC bus capacitor 42 through the DC bus capacitor pre-charging circuit 41.

[0034] S2: Measure the grid-connected voltage u of inverter 4 a 、Filter capacitor voltage u C , filter inductor current i L , further processed to obtain the input signal of the hysteresis controller, including the filter capacitor voltage reference value u cref (u cref =u amp *cosθ*m, where u amp is the fundamental component amplitude of the inverter grid-connected point voltage, θ is the phase of the fundamental component of the inverter grid-connected point voltage, m is the slow start coefficient that gradually increases from 0 to 1), filter capacitor current i C , filter inductor current i L .

[0035] S3: Figure 2 As shown, the DC / AC inverter circuit 45 is driven by the output signal of the hysteresis controller, so that the filter capacitor voltage enters a voltage following state, that is, the filter capacitor voltage phase is synchronized with the grid-connected point voltage fundamental component phase, and the filter capacitor voltage amplitude slowly rises from 0 to within the range of 75% to 90% of the grid-connected point voltage fundamental component amplitude.

[0036] S4: Figure 3 As shown, whether the inverter 4 enters the normal working mode is determined according to the preset second AC relay s2 closing judgment logic, specifically:

[0037] S41, calculate the filter capacitor voltage u C and the filter capacitor voltage reference value u cref The difference Err = u C -u cref ;

[0038] S42, if ucref If it is within the preset range (-150V, 150V), jump to the next step, otherwise jump to S2;

[0039] S43, if Err is within the preset range (-15V, 15V), jump to the next step, otherwise jump to S2;

[0040] S44, if m=u cref / (u amp If cosθ) is greater than the preset threshold 0.8, jump to the next step, otherwise jump to S2;

[0041] S45, if u C If the amplitude reaches the preset threshold value 0.005s within one power frequency cycle, the second AC relay s2 which has been in the disconnected state is closed, the first AC relay s1 is disconnected, and the inverter 4 enters the normal working mode; otherwise, jump to S2.

[0042] The above embodiments are descriptions of specific implementation methods of the present invention rather than limitations of the present invention. Technical personnel in the relevant technical field may make various changes and modifications to obtain corresponding equivalent technical solutions without departing from the spirit and scope of the present invention. Therefore, all equivalent technical solutions should be included in the patent protection scope of the present invention.

Claims

1. An inverter, characterized in that: The inverter includes a DC bus capacitor, a DC bus capacitor pre-charging circuit, a first AC relay, a DC / AC inverter circuit including a current controller module, a filter capacitor, a filter inductor, and a second AC relay; The connection relationship is as follows: the DC bus capacitor pre-charging circuit, the DC bus capacitor, and the DC / AC inverter circuit are connected in parallel in sequence; one end of the filter inductor is connected to the DC / AC inverter circuit, and the other end is connected to one end of the filter capacitor and one end of the second AC relay; the other end of the filter capacitor is connected to the DC / AC inverter circuit and is connected in parallel to one end of the AC power grid as an inverter; one end of the first AC relay is connected to the DC bus capacitor pre-charging circuit, and the other end is connected to the other end of the second AC relay and is connected in parallel to the other end of the AC power grid as an inverter.

2. The inverter according to claim 1, characterized in that: The current controller is a hysteresis controller.

3. A method for starting an inverter, characterized in that: The method comprises the following steps: S1: Send a start-up command to the inverter, and the first AC relay is closed; S2: Measure the inverter grid connection point voltage, filter capacitor voltage, and filter inductor current, and further process them to obtain the input signal of the current controller; S3: The output signal of the current controller drives the inverter circuit, so that the filter capacitor voltage enters the voltage following state, that is, the filter capacitor voltage phase is synchronized with the phase of the fundamental component of the grid-connected point voltage, and the filter capacitor voltage amplitude slowly rises from 0 and remains within the range of a% to b% of the fundamental component amplitude of the grid-connected point voltage; S4: Determine whether the inverter enters the normal working mode according to the preset second AC relay closing judgment logic, specifically: S41, calculate the filter capacitor voltage u C and the filter capacitor voltage reference value u cref The difference Err = u C -u cref ; S42, if u cref If it is within the preset range, jump to the next step, otherwise jump to S2; S43, if Err is within the preset range, jump to the next step, otherwise jump to S2; S44, if u cref / (u amp cosθ) is greater than the preset threshold, jump to the next step, otherwise jump to S2; where u amp is the amplitude of the fundamental component of the voltage at the inverter grid connection point, θ is the phase of the fundamental component of the voltage at the inverter grid connection point; S45, if u C If the amplitude reaches the preset threshold value for a period of time within one power frequency cycle, the second AC relay is closed, the first AC relay is disconnected, and the inverter enters the normal working mode; otherwise, it jumps to S2.

4. The method for starting an inverter according to claim 3, characterized in that: In S3, a=75, b=90.

5. The method for starting an inverter according to claim 3, characterized in that: The second AC relay is in an open state until it is closed in step S4.

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