阅读说明：本技术 逆变器及逆变器的软启动方法 (Inverter and soft start method thereof ) 是由 魏琪康 徐飞冬 朱选才 于 2019-11-05 设计创作，主要内容包括：本发明揭露一种逆变器及逆变器的软启动方法。所述逆变器电性连接于一直流电源与一电网之间,所述逆变器包括有主逆变电路、滤波器、继电器以及控制器,并且所述主逆变电路通过所述继电器与所述电网电性耦接,所述控制器分别与所述主逆变电路和所述继电器电性连接,所述软启动方法包括：采样所述电网的电网电压；计算所述控制器的输出电压,并根据所述控制器的输出电压、所述电网电压以及一虚拟阻抗计算得到虚拟电流；将所述虚拟电流作为反馈信号进行电流闭环控制,并待控制到达稳态后,发出继电器闭合信号以闭合所述继电器。本发明可以在不增加启动电阻等任何硬件电路的前提下,实现逆变器的软启动。(The invention discloses an inverter and a soft start method of the inverter. The inverter is electrically connected between a direct current power supply and a power grid, the inverter comprises a main inverter circuit, a filter, a relay and a controller, the main inverter circuit is electrically coupled with the power grid through the relay, the controller is respectively electrically connected with the main inverter circuit and the relay, and the soft start method comprises the following steps: sampling the grid voltage of the power grid; calculating the output voltage of the controller, and calculating to obtain a virtual current according to the output voltage of the controller, the power grid voltage and a virtual impedance; and performing current closed-loop control by taking the virtual current as a feedback signal, and sending a relay closing signal to close the relay after the control reaches a stable state. The invention can realize the soft start of the inverter on the premise of not increasing any hardware circuits such as a starting resistor and the like.)

1. A soft start method of an inverter, the inverter being electrically connected between a DC power source and a power grid, the inverter comprising a main inverter circuit, a filter, a relay and a controller, the main inverter circuit being electrically coupled to the power grid through the relay, the controller being electrically connected to the main inverter circuit and the relay, respectively, the soft start method comprising:

sampling the grid voltage of the power grid;

calculating the output voltage of the controller, and calculating to obtain a virtual current according to the output voltage of the controller, the power grid voltage and a virtual impedance;

and performing current closed-loop control by taking the virtual current as a feedback signal, and sending a relay closing signal to close the relay after the control reaches a stable state.

2. The soft-start method of an inverter according to claim 1, further comprising:

sampling the inverter side current and judging whether the inverter side current is larger than a threshold value,

if the current is larger than the preset value, selecting the inverter side current as a feedback signal to carry out current closed-loop control;

and if the current is smaller than the preset value, continuing to perform current closed-loop control by taking the virtual current as a feedback signal.

3. The soft-start method of an inverter according to claim 2, further comprising:

and comparing the virtual current serving as a feedback signal with a current command reference to generate a current error, and determining the output voltage of the controller corresponding to the current error through a current regulator.

4. The soft-start method of an inverter according to claim 3, further comprising:

sampling the direct current bus voltage, performing voltage loop control according to the direct current bus voltage and a direct current bus reference value, and taking the output of the voltage loop as the current instruction reference.

5. The soft-start method of an inverter according to claim 4, further comprising:

and carrying out amplitude limiting control on the output of the voltage loop.

6. The soft-start method of an inverter according to claim 1, wherein the virtual current is calculated using the following formula:

wherein, I_vFor said virtual current, V_eIs the output voltage of the controller, V_gFor the grid voltage, Z_vIs the virtual impedance.

7. The soft-start method of an inverter according to claim 4, wherein the inverter is a single-phase inverter, and a single-phase grid is coupled through the relay.

8. The soft-start method of an inverter according to claim 4, wherein the inverter is a three-phase inverter, and a three-phase grid is coupled through the relay.

9. The soft-start method of an inverter according to claim 8, further comprising:

and converting the power grid voltage and the inverter side current into voltage and current in a dq synchronous rotation coordinate system through coordinate transformation to control.

10. The soft-start method of an inverter according to claim 9, further comprising:

and taking the output of the voltage loop as a d-axis current command reference, and setting the q-axis current command reference as a preset value.

11. The soft-start method of an inverter according to claim 2, wherein a value of the threshold is larger than a maximum normal current on the inverter side when the relay is not closed.

12. The soft-start method of an inverter according to claim 1, wherein the controller issues the relay closing signal to close the relay after a delay time.

13. The soft-start method of an inverter of claim 3, wherein the controller issues the relay close signal to close the relay after the current error is less than an error threshold.

14. The utility model provides an inverter, inverter electric connection is between a DC power supply and a electric wire netting, inverter is including main inverter circuit, wave filter, relay and controller, main inverter circuit pass through the relay with the electric wire netting electric coupling, the controller respectively with main inverter circuit with relay electric connection, its characterized in that, the controller includes:

the sampling unit is used for sampling the grid voltage of the power grid;

the virtual current calculating unit is used for calculating the output voltage of the controller and calculating a virtual current according to the output voltage of the controller, the power grid voltage and a virtual impedance;

and the current control unit is used for carrying out current closed-loop control by taking the virtual current as a feedback signal and sending a relay closing signal to close the relay after the control reaches a steady state.

15. The inverter of claim 14, wherein the sampling unit is further configured to sample an inverter-side current;

the controller also comprises a feedback quantity selection unit for judging whether the current on the side of the inverter is larger than a threshold value,

if the current is larger than the preset value, selecting the inverter side current as a feedback signal to carry out current closed-loop control;

and if the current is smaller than the preset value, selecting the virtual current as a feedback signal to carry out current closed-loop control.

16. The inverter of claim 15, wherein the current control unit is further configured to compare the virtual current as a feedback signal to a current command reference to generate a current error, and determine the output voltage of the controller corresponding to the current error via a current regulator.

17. The inverter according to claim 16, wherein the sampling unit is further configured to sample a dc bus voltage of the inverter, and the controller further comprises a voltage control unit configured to perform voltage loop control according to the dc bus voltage and a dc bus reference value, and use an output of the voltage loop as the current reference command.

18. The inverter of claim 17, wherein the voltage control unit further comprises a clipping unit.

19. The inverter according to claim 14, wherein the virtual current calculating unit calculates the virtual current according to the following formula:

wherein, I_vFor said virtual current, V_eIs the output voltage of the controller, V_gFor the grid voltage, Z_vIs the virtual impedance.

20. The inverter of claim 17, wherein the inverter is a single-phase inverter, and wherein a single-phase grid is coupled to the inverter via the relay.

21. The inverter of claim 17, wherein the inverter is a three-phase inverter, and wherein a three-phase grid is coupled through the relay.

22. The inverter of claim 21, wherein the controller further comprises:

and the coordinate conversion unit is used for converting the grid voltage and the inverter side current into voltage and current in a dq synchronous rotation coordinate system through coordinate conversion and controlling the voltage and the current.

23. The inverter of claim 22, wherein the voltage control unit is further configured to reference the output of the voltage loop as a d-axis current command reference and to set the q-axis current command reference to a predetermined value.

24. The inverter of claim 15, wherein the threshold value is greater than a maximum normal current on the inverter side when the relay is not closed.

25. The inverter of claim 14, wherein the controller issues the relay close signal to close the relay after a delay time.

26. The inverter of claim 16, wherein the controller issues the relay close signal to close the relay after the current error is less than an error threshold.

27. The inverter of claim 14, wherein the filter is an L filter, an LC filter, or an LCL filter.

Technical Field

The invention relates to an inverter and a soft start method of the inverter.

Background

In a grid-connected inverter, when a relay is closed, a voltage difference exists between a voltage on a capacitor between an inductor and the relay and a power grid voltage, a large impact current is generated, and device damage and interference on a control system can be caused. In order to ensure the safe starting of the device, the voltage difference between the front and the rear of the relay needs to be kept at a small value, namely, the synchronization of the voltage on the capacitor and the voltage of the power grid is realized.

In the existing method, the impact current is reduced by a method of connecting resistors in series, but the method increases the hardware cost and does not meet the safety requirements of the inverter. Another method is to generate a voltage waveform similar to the power grid voltage on a capacitor in front of a relay through power grid voltage feedforward control, but the method is open-loop control and is difficult to ensure that the voltage difference is constant and zero. The current will increase rapidly in a short time after the relay is closed, and the drive needs to be blocked in time. However, the actual closing time of the three relays of the three-phase inverter is different, and the inverter switching action is required to be maintained for a relatively long time to ensure that the voltage difference between the two ends of the three relays is small when all the relays are closed. There are other methods of closed loop control, but these require the acquisition of the voltage in front of the relay, increasing the hardware cost. In addition, the existing soft start method is less related to energy control, and the fluctuation of the bus voltage is uncontrollable. For example, the voltage of a panel is high in case of strong light, and overvoltage protection is easily triggered if the voltage of a bus fluctuates upwards; when the light is weak, the energy of the panel is insufficient, and the starting is difficult if the bus voltage drops instantaneously.

Therefore, it is very necessary to find a method for easily and effectively solving the problem of synchronization of the front and rear voltages of the relay and controlling the soft start of the inverter.

Disclosure of Invention

In view of the above, an object of the present invention is to provide an inverter and a soft start method for the inverter, which can realize the soft start of the inverter without adding any hardware circuit such as a start resistor or a voltage acquisition circuit.

Another object of the present invention is to provide an inverter and a soft start method of the inverter, which can adapt to the situation that the actual closing time of three relays is inconsistent. Meanwhile, the method can control energy and prevent starting failure caused by bus voltage fluctuation.

In order to achieve the above object, the present invention provides a soft start method of an inverter, the inverter being electrically connected between a dc power source and a power grid, the inverter including a main inverter circuit, a filter, a relay and a controller, the main inverter circuit being electrically coupled to the power grid through the relay, the controller being electrically connected to the main inverter circuit and the relay, respectively, the soft start method comprising:

sampling the grid voltage of the power grid;

calculating the output voltage of the controller, and calculating to obtain a virtual current according to the output voltage of the controller, the power grid voltage and a virtual impedance;

and performing current closed-loop control by taking the virtual current as a feedback signal, and sending a relay closing signal to close the relay after the control reaches a stable state.

In order to achieve the above object, the present invention further provides an inverter electrically connected between a dc power source and a power grid, the inverter including a main inverter circuit, a filter, a relay, and a controller, the main inverter circuit being electrically coupled to the power grid through the relay, the controller being electrically connected to the main inverter circuit and the relay, respectively, wherein the controller includes:

the sampling unit is used for sampling the grid voltage of the power grid;

the virtual current calculating unit is used for calculating the output voltage of the controller and calculating a virtual current according to the output voltage of the controller, the power grid voltage and a virtual impedance;

and the current control unit is used for carrying out current closed-loop control by taking the virtual current as a feedback signal and sending a relay closing signal to close the relay after the control reaches a steady state.

According to the soft start method of the inverter based on the virtual current control, hardware circuits such as a start resistor or a voltage acquisition circuit do not need to be added, and the soft start method can adapt to the situation that the closing time of the three-phase relay is inconsistent. The invention can also control energy, and effectively prevent the starting failure caused by the voltage fluctuation of the bus.

The above description will be described in detail by embodiments, and further explanation will be provided for the technical solution of the present invention.

Drawings

In order to make the aforementioned and other objects, features, and advantages of the invention, as well as others which will become apparent, reference is made to the following description taken in conjunction with the accompanying drawings in which:

fig. 1 is a schematic structural diagram of an inverter according to a preferred embodiment of the invention;

fig. 2 is a schematic flow chart of a soft start method of an inverter according to a preferred embodiment of the invention;

FIG. 3 is a control block diagram of a three-phase inverter of the present invention controlled using virtual current;

FIG. 4 is a control block diagram of a single-phase inverter of the present invention controlled using virtual current;

FIG. 5 is a diagram illustrating the selection of virtual impedances Z according to the open loop gain_vOpen-loop bode plots for inductive and resistive;

FIG. 6 is a schematic diagram of the present invention applied to a single-phase inverter for soft start;

Detailed Description

For a better understanding and completeness of the description of the present invention, reference is made to the appended drawings and various embodiments described below in which like reference numerals represent the same or similar elements. In other instances, well-known elements and steps have not been described in detail in order to avoid unnecessarily obscuring the present invention. In addition, for the sake of simplicity, some conventional structures and elements are shown in the drawings in a simple schematic manner.

As shown in fig. 1, a structure of an inverter 100 according to a preferred embodiment of the present invention is shown. The inverter 100 is electrically connected between a dc power source 200 and a power grid 300. The dc power source may be a directly connected voltage source, such as an energy storage cell, a PV panel, etc., or may be a preceding stage circuit having a dc output, such as a two-stage photovoltaic inverter. The inverter 100 mainly includes a main inverter circuit 11, a filter 12, a relay 13, and a controller 14. The main inverter circuit 11 is electrically coupled to the power grid 300 through the relay 13, the filter 12 is electrically connected between the main inverter circuit 11 and the relay 13, and is configured to filter the voltage and the current output by the inverter 100, and the controller 14 is electrically connected to the main inverter circuit 11 and the relay 13, and is configured to control the operating state of the main inverter circuit 11 and the on/off of the relay 13 according to actual requirements.

In this embodiment, the main inverter circuit 11 may include a dc bus capacitor C_busAnd a switching leg 111. DC bus capacitor C_busAnd is electrically coupled between the two output terminals of the dc power supply 200 and electrically connected to the two input terminals of the switching arm 111. In a three-phase system, the switching leg 111 may be, for example, a three-phase full-bridge topology; in a single-phase system, switching leg 111 may be, for example, a single-phase full-bridge topology. However, it is to be understood that the main inverter circuit is not limited to the above topology, and may have other topologies, which are not intended to limit the invention.

In the present embodiment, inverter 100 is a three-phase inverter, and grid 300 is a three-phase grid, i.e., including a-phase, B-phase, and C-phase. The filter 12 is exemplified by an LC filter, i.e. comprising a filter inductance L_fAnd a filter capacitor C_f. The relay 13 is a three-phase relay K. Three output ends of the main inverter circuit 11 are connected to three phases of the power grid 300 through LC filters and three-phase relays K, respectively. However, it is understood that inverter 100 may also be a single-phase inverter, and accordingly, grid 300 may also be a single-phase grid (as shown in fig. 4). Also, the filter 12 may be other types of filters, such as an L filter or an LCL filter, which are not intended to limit the present invention.

In the present invention, the controller 14 includes at least a sampling unit 141, a virtual current calculation unit 142, and a current control unit 143. The sampling unit 141 is used for sampling the grid voltage V of the grid 300_g. The virtual current calculating unit 142 is used for calculating the output voltage V of the controller 14_eAnd according to the output voltage V of the controller_eThe network voltage V_gAnd a virtual impedance Z_vCalculating a virtual current I_v. The current control unit 143 is used for controlling the virtual current I_vAnd performing current closed-loop control as a feedback signal, and sending a relay closing signal to close the relay 13 after the control reaches a steady state.

In the present invention, the virtual current calculating unit 142 may calculate the virtual current I according to the following equation 1_v：

Wherein, I_vIs a virtual current, V_eIs the output voltage of the controller, V_gFor the mains voltage, Z_vIs a virtual impedance.

Preferably, in the present embodiment, the current control unit 143 is further configured to control the dummy current I_vAs a feedback signal and a current command reference I_refThe comparison is performed to generate a current error, and the output voltage V of the controller 14 corresponding to the current error can be determined by the current regulator_e。

The so-called "steady state" criterion may be, for example, the virtual current I_vApproximately equal to current command reference I_refIt may be considered to reach a steady state after waiting a sufficient period of time. In other embodiments, the output voltage V of the controller 14_eFor example, a modulation gain of PWM, the magnitude of which is equal to the network voltage V_gAnd (4) the equivalent.

Preferably, in the present embodiment, the sampling unit 141 can also be used for sampling the inverter-side current I_invIn which the inverter-side current I_invFor example, the current at the output terminal of the main inverter circuit 11, or the filter inductance L of the filter 12_fOf the inductor current I_L. The controller 14 further includes a feedback quantity selection unit 144 for determining the inverter-side current I_invWhether the current is larger than a threshold value A or not, if so, selecting the side current I of the inverter_invCurrent closed-loop control is carried out as a feedback signal; if the current is less than the preset value, selecting to convert the virtual current I into the virtual current I_vAnd performing current closed-loop control as a feedback signal. The value of the threshold a is greater than the maximum normal current on the inverter side when the relay 13 is not closed. The selection principle of the threshold value A is to ensure that when the relay 13 is not closed, the controller does not make a misjudgment when comparing the magnitude of the inductive current with the threshold value A, so that the virtual current control is exited in advance. Assuming that the virtual current control reaches steady state, the peak value of the current at the inverter side is I_pAfter the relay is closed, the current tends to increase due to the difficulty in achieving complete balance in control (or an unbalanced state may be actively generated during controller design). If the virtual current control is continuously adopted, the maximum value of the current exceeds I_pAnd is increasing. So that a timely switch to true current control is required. Otherwise, if the set threshold is smaller than I_pIf the relay is not closed, the controller is already switched to real current control. A phenomenon occurs in which the current is controlled to a given value, but the voltage is not synchronized with the grid. In summary, it is necessary to determine whether the relay is closed according to the relationship between the current and the threshold a, and then determine whether to exit the virtual current control.

Wherein, for three-phase inverter, feedback quantity selection unit 144 selects inverter-side current I of each phase_invi(where I is a, b, c) is compared with a threshold value a if the inverter-side current I of any one phase is larger than the threshold value a_inviIf the current is larger than the threshold A, the current I on the side of the inverter is converted into the current I_invi(where I ═ a, b, c) is controlled as a feedback quantity, otherwise the corresponding virtual current I is used_vControl is performed as a feedback amount.

Further, in the application of the photovoltaic inverter, when the sunlight is strong, the output voltage of the photovoltaic panel is high, so that the voltage of a direct current bus fluctuates upwards to trigger overvoltage protection easily; when sunlight is weak, the energy of the photovoltaic panel is insufficient, and the direct current bus voltage is likely to drop instantly, so that starting difficulty is easily caused. Therefore, in the present embodiment, the sampling unit 141 can also be used for sampling the output voltage of the dc power supply 200, such as the dc bus voltage V of the inverter 100_bus. The controller 14 further comprises a voltage control unit 145, which may be, for example, a dc bus voltage loop, which may be used for controlling the voltage according to the dc voltageBus voltage V_busAnd a DC bus reference value V_busrefPerforming voltage loop control, and using the output of the voltage loop as a current reference instruction I_ref. In other embodiments, the voltage control unit 145 may further include a clipping unit (not shown) to clip the output of the voltage loop.

In a three-phase inverter control system, as may be combined with fig. 3, the controller 14 may further include a first coordinate transformation unit operable to transform the grid voltage V_gInverter side current I_invThe voltage and current in the dq synchronous rotation coordinate system are converted from an abc three-phase coordinate system through coordinate transformation, and control is performed. The voltage control unit 145 may also be used to reference the output of the voltage loop as a d-axis current command reference I_drefAnd setting a q-axis current command reference I_qrefIs a predetermined value, for example, set to 0. Further, the controller 14 further includes a second coordinate conversion unit, which is configured to convert the control results in the d-axis and the q-axis to the abc three-phase coordinate system for pulse width modulation.

Further, the controller 14 sends a switching action signal to the switching arm 111 of the main inverter circuit 11 according to the pulse width modulation result to control the on/off of the corresponding switch. As shown in fig. 1, the controller 14 may further include a modulation unit 148, for example, wherein the modulation unit 148 may be an SVPWM modulation control, for example, and may be configured to modulate the corresponding voltage to generate the corresponding driving to drive the corresponding switch. Further, the controller 14 may further include a phase locking unit 146, for example, and the phase locking unit 146 may be configured to perform phase locking according to the grid voltage.

In the present invention, the controller 14 may, for example, send out a corresponding relay action signal (including a relay closing signal and a relay opening signal) through a relay action control unit 147 to control the corresponding relay to perform a corresponding action, such as sending out a relay closing signal to close the relay 13 or sending out a relay opening signal to open the relay 13. Preferably, in the present invention, the relay closing signal may be sent by the controller 14 to close the relay 13 after a delay time, for example, after the soft start control is performed; alternatively, the controller 14 may send a relay closing signal to close the relay 13 after the current error is smaller than an error threshold, that is, whether the control reaches the steady state may be determined by the above method.

Accordingly, in conjunction with fig. 1, the soft start method of the inverter of the present invention may include, for example:

step S1, sampling the grid voltage V of the grid 300_g；

Step S2, calculating the output voltage V of the controller 14_eAnd according to the output voltage V of the controller 14_eVoltage V of the power grid_gAnd a virtual impedance Z_vCalculating to obtain a virtual current I_v；

Step S3, converting the virtual current I_vAnd performing current closed-loop control as a feedback signal, and sending a relay closing signal to close the relay 13 after the control reaches a steady state.

Preferably, the soft start method of the inverter may further include: sampling inverter side current I_invAnd judging the inverter side current I_invWhether the current is larger than a threshold value A or not, if so, selecting the side current I of the inverter_invCurrent closed-loop control is carried out as a feedback signal; if the current is less than the preset value, continuing to apply the virtual current I_vAnd performing current closed-loop control as a feedback signal.

Preferably, the soft start method of the inverter may further include: will make a virtual current I_vAs a feedback signal and a current command reference I_refA comparison is made to produce a current error, which is determined by the current regulator as the output voltage V of the controller 14 to which the current error corresponds_e。

Preferably, the soft start method of the inverter may further include: sampling DC bus voltage V_busAccording to the DC bus voltage V_busAnd a DC bus reference value V_busrefPerforming voltage loop control, and using the output of the voltage loop as the current command reference I_ref。

Preferably, the soft start method of the inverter may further include: the output of the voltage loop is clipped.

Preferably, the soft start method of the inverter may further include: will the network voltage V_gInverter side current I_invAnd converting the coordinate into voltage and current in a dq synchronous rotation coordinate system through coordinate transformation to control. For example, the output of the voltage loop may be referred to as the d-axis current command reference I_drefAnd the current instruction of the q axis is referred to I_qrefSet to a predetermined value.

As shown in fig. 2, it shows a soft start method of the inverter 100 of fig. 1, which mainly includes:

step 201, sampling the grid voltage V in real time_g(e.g. instantaneous voltage), DC bus voltage V_busAnd the inverter side current I_inv(e.g., inductor current).

Step 202, taking the output of the direct current bus voltage loop as a current instruction reference I of current closed-loop control_ref。

Step 203, synthesize the virtual impedance Z_vAnd the output voltage V of the controller_eCalculating a virtual current I_vI.e. I_v＝(V_e-V_g)/Z_v. In this embodiment, to simplify the calculation, for example, the virtual impedance Z is selected_vIs 2 omega. Of course, it will be understood that the virtual impedance Z_vThe value of (A) is not limited thereto. For example, the same impedance value as the actual inductance may be selected.

Step 204: will make a virtual current I_vAnd performing current closed-loop control as a feedback signal, and sending a relay closing signal to close the relay after the control reaches a stable state.

In this step 204, the dummy current I is controlled in preparation for starting the inverter_vAfter entering steady state, V_e＝I_v*Z_v+V_gVirtual current I_vThe resulting pressure drop I_v*Z_vRelative grid voltage V_gCan be ignored, i.e. V_eAnd V_gSynchronization is achieved. V_eCapacitor C in front of relay after filter_fUpper generation voltage V_inv. Depending on the characteristics of the filter, this voltage V_invApproximately equal to the grid voltage V_g. Thus realizingThe voltage difference between the front and the back of the relay is small. A relay close signal may be issued by the relay action control unit 147 to close the relay at this time.

Step 205, determining whether the inductor current (for example, an instantaneous value of the inductor current) of the inverter is greater than a threshold a, if yes, performing step 205; if no, go to step 206. And if the inverter is a three-phase inverter, judging whether the inductive current of any phase of the inverter is larger than a threshold value A.

A delay typically elapses from the time the relay is signaled to close until the relay is actually closed, and for a three-phase inverter, the three-phase relay may not be fully synchronized, so that the current typically increases rapidly after the relay is closed.

And step 206, if the inductive current is larger than the threshold value A, taking the inductive current as a feedback signal, and continuing the current closed-loop control.

Step 207: and if the inductive current is smaller than the threshold A, taking the virtual current as a feedback signal, and continuing the current closed-loop control.

Step 208: and waiting for the three-phase relays to complete the closing, and completing the soft start process.

In step 202, the dc bus voltage V may be further adjusted by an adjuster of the voltage control unit 145 (e.g., a dc bus voltage loop)_busControl is performed with the control result as the current command reference I_ref. In practice, the command for the dc bus voltage loop may be set slightly higher than the current real bus voltage. It should be noted that, when the virtual current control is adopted, the dc bus voltage cannot be actually influenced. Therefore, the dc bus voltage loop may output a continuously increasing negative value (if it is defined that the current flowing to the grid is positive) after passing through, for example, the PI regulator, and it is necessary to limit the amplitude of the dc bus voltage loop, that is, the output of the voltage loop needs to be limited. The dc bus voltage loop only functions when switching to inverter side current (inductor current) control. Of course, it can be understood that whether the dc bus voltage loop functions or not may be achieved by limiting, or switching the current command by a program, and switching from a fixed current command to the dc bus voltage loopThe output of the voltage loop is achieved.

In step 203, for the three-phase inverter, the voltage quantities of the d-axis and the q-axis can be respectively substituted into formulas to calculate virtual current values of the d-axis and the q-axis. For a single-phase inverter, the instantaneous value of the virtual current can be calculated by directly substituting a formula.

In step 205, the threshold a should be greater than the maximum normal current of the inverter side when the relay is not closed, that is, the maximum value of the current generated by the voltage output by the inverter on its own LC filter, so as to ensure that the relay is not closed and will not switch to the operating state in which the inductor current is used as the feedback signal. For a three-phase inverter, judging whether the maximum value of the inductive current of any phase is greater than a threshold A; for the single-phase inverter, whether the instantaneous value of the inductive current is larger than the threshold value A can be directly judged.

As shown in fig. 3, a control block diagram for soft start of the three-phase inverter of the present invention is shown. Wherein, three-phase network voltage V is sampled_gInverter side current I_inv(inductor Current I)_L) Firstly, it is converted from abc three-phase coordinate system to a DC signal in dq rotating coordinate system by coordinate transformation, for example, the sampled grid voltage V of a three-phase grid_gCan be transformed into a voltage V in a dq rotating coordinate system after coordinate transformation_g(dq)And the sampled inductor current I_L(i.e., inverter-side current) can be converted into a current I in dq rotation coordinate system by coordinate transformation_L(dq)。

Wherein the network voltage V_gCan be used for phase locking and calculating the virtual current, such as the virtual current I calculated by the virtual current calculating unit 142 in FIG. 3_v(dq)Wherein the d-axis virtual current I_v(d)Can be calculated by calculating the output voltage V of the d-axis_e(d)And the network voltage V_g(d)Is divided by the virtual impedance Z_vObtaining; virtual current I of q-axis_v(q)The algorithm of (c) is consistent with the d-axis.

During soft start, the dummy current I is applied_v(dq)As a feedback control amount. When a relay closing signal is sent out, the current I of the inductor is adjusted_LCompared with a threshold A and when the condition is satisfiedAnd switching to the inductor current control as a feedback signal. If the inductive current I of any phase_LIf the instantaneous value of (A) is greater than (A), the inductive current I is used_LAnd controlling as a feedback signal, otherwise, controlling by adopting the virtual current as the feedback signal. In other words, if I_L> A, then I_(dq)＝I_L(dq)I.e. using the inductor current I_L(dq)As a feedback signal I_(dq)(ii) a If I_LLess than or equal to A, then I_(dq)＝I_v(dq)I.e. using a virtual current I_v(dq)As a feedback signal I_(dq)。

In fig. 3, the dc bus voltage loop is controlled by a PI regulator, and its output is referred to as a d-axis current command. For example, it can be based on the DC bus voltage V_busAnd a DC bus reference value V_busrefOutputting after PI regulation, and taking the output quantity as the current instruction reference I of the d axis_dref. In addition, the q-axis current command reference may be set to 0, i.e., I_qrefWhen the value is 0, the output voltage V may be trimmed by taking a value near 0_eAnd the network voltage V_gThe phase angle difference between them. In this embodiment, the dc bus voltage loop functions after the virtual current control is switched to the actual inductive current control, and the bus voltage can be stabilized.

Further, a feedback signal I_(dq)With a current command reference (e.g. including I)_dref、I_qref) Comparing to generate a current error, and performing current loop PI regulation through a current regulator to determine the output voltage V of the controller corresponding to the current error_e(dq). Output voltage V_e(dq)Continuously changes into an output voltage V in a three-phase coordinate system after coordinate transformation_e(abc)And by amplitude variation (e.g. 2/V)_bus) And after SVPWM modulation, the output controls the on-off of a switching tube of the main inverter circuit. It should be understood that in this embodiment, the voltage loop and the current loop adopt PI control, and in other embodiments, other regulators, such as PR control, etc., may be adopted.

In the present embodiment, the virtual impedance Z_vEither resistive or inductive. The inductive virtual impedance, i.e. the effect of the analog inductance,the effect of current slow increase can be achieved, and in addition, the high frequency band in the bode diagram has attenuated gain, which is beneficial to design of control parameters. The calculation process of the resistive virtual impedance is simple, and the resistive virtual impedance can be selected when the original controller is simple and the stability can be ensured.

Fig. 4 shows a control block diagram of the present invention applied to a single-phase inverter for soft start. The difference between the single-phase inverter shown in fig. 4 and the three-phase inverter shown in fig. 3 is that the single-phase inverter does not need to perform coordinate conversion. According to the difference, some corresponding adjustment needs to be performed on the control block diagram, for example, the output of the direct-current bus voltage loop is taken as the amplitude of the current, and the current can be used as the instruction of the current only by multiplying by sin theta; and the control of the current loop can adopt PI control or PR control, and the scheme is not limited to this.

Fig. 5 shows a control block diagram for control with a virtual current. Wherein G is_IAnd(s) is a controller of a current loop, T is time delay, and the three-phase inverter adopts PI control in the embodiment. Then, according to fig. 5, the transfer function of the open loop gain is known as:

FIG. 6 shows the gain G according to the open loop_open(s) selecting the virtual impedances Z respectively_vThe open loop bode plot for inductive and resistive. As can be seen from the figure, for the resistive virtual impedance, the larger the resistance is, the smaller the gain is, and the slower the dynamic speed is, but the stability is easier to ensure, so a compromise value can be selected as required. For a resistive virtual impedance, it is generally chosen to set the same parameters as the actual filter inductance. In this embodiment, a resistive virtual impedance is selected, and a virtual impedance Z is set_vIs 2 ohms. It should be understood that the virtual impedance may be calculated according to actual conditions, and the present application is not limited thereto.

By adopting the soft start method of the inverter based on the virtual current control, the inverter can be safely started without adding hardware circuits such as a starting resistor or a voltage acquisition circuit and the like. In addition, the invention can be more suitable for the situation that the closing time of the three-phase relay is inconsistent, can control the energy and effectively prevent the starting failure caused by the voltage fluctuation of the bus.

Although the present invention has been described with reference to the above embodiments, it should be understood that various changes and modifications can be made therein by those skilled in the art without departing from the spirit and scope of the invention.