Inverter device and active island detection method thereof

Abstract

The present application discloses an inverter device and an active island detection method thereof. The inverter device receives a direct current voltage and converts it into an alternating current, and transmits the alternating current to a power grid. First, the alternating current voltage is band-pass filtered to obtain a passband frequency of the alternating current voltage. When the passband frequency is less than a critical frequency value, a first control frequency value is generated according to an alternating positive frequency shift coefficient and a negative frequency shift coefficient. When the passband frequency is not less than the critical frequency value, a second control frequency value is continuously generated according to an initial frequency shift coefficient. Finally, the frequency of the alternating current is adjusted according to one of the first control frequency value and the second control frequency value, the alternating current, the alternating current voltage, a real power command and an imaginary power command.

Description

Technical Field

[0001] This invention relates to an active islanding detection technology, and more particularly to an inverter device and its active islanding detection method. Background Technology

[0002] Islanding detection technology is an essential technology in power generation equipment. It is mainly responsible for handling energy resonance caused by power generation equipment when the area is disconnected from the grid, which can prevent the equipment from stopping power generation normally and lead to related power safety issues (e.g., electric shock to maintenance personnel). In the standard islanding test procedure, a power outage is simulated under regional resonance of the power equipment. Under this resonance condition, active islanding detection is usually required to achieve the protection effect. Active islanding detection refers to detecting whether the equipment is disconnected from the grid by injecting disturbance signals.

[0003] While injecting disturbances can enhance the detection capability under system resonance, it can also lead to a decline in power quality because it actively injects disturbances into the mains power. Moreover, the power quality is subject to stricter quality specifications in the mains power grid connection regulations (such as EN50549). In addition to the requirement of 5% for harmonics and total harmonic distortion (THD), the power / virtual power setting accuracy must be less than 2%. Active disturbances can easily cause these conditions to fail to meet the grid connection requirements simultaneously.

[0004] Therefore, the present invention addresses the aforementioned problems by proposing an inverter device and its active islanding detection method to solve the problems caused by the prior art. Summary of the Invention

[0005] The purpose of this invention is to provide an inverter device and its active islanding detection method, which achieves the effect of actively detecting unplanned islanding systems while minimizing injected disturbances, smoothly allowing the mains power to disconnect and switch to standalone mode, and improving the control accuracy of real / virtual power under grid-connected conditions, in compliance with grid-connected regulations.

[0006] In one embodiment of the present invention, an active islanding detection method is provided, which is applied to an inverter device coupled to a power grid and a load. The power grid generates an AC voltage, and the inverter device receives a DC voltage, converts it into an AC current, and transmits the AC current to the power grid. The active islanding detection method includes the following steps: bandpass filtering the AC voltage to obtain the passband frequency of the AC voltage; determining whether the passband frequency is less than a critical frequency value; if so, generating a first control frequency value alternately according to a positive frequency shift coefficient and a negative frequency shift coefficient, wherein the first control frequency value is associated with the instantaneous frequency value of the AC voltage and the passband frequency; and if not, continuously generating a second control frequency value according to an initial frequency shift coefficient, wherein the second control frequency value is associated with the instantaneous frequency value of the AC voltage and the passband frequency; and adjusting the frequency of the AC current according to one of the first control frequency value and the second control frequency value, the AC current, the AC voltage, a real power command, and a virtual power command.

[0007] In one embodiment of the present invention, the first control frequency value and the second control frequency value are generated with different periods, and the period corresponding to the first control frequency value is greater than the period corresponding to the second control frequency value.

[0008] In one embodiment of the present invention, the first control frequency values ​​corresponding to the positive frequency shift coefficient and the negative frequency shift coefficient are generated in the Nth cycle and the 2Nth cycle of the AC voltage, respectively, where N is a positive integer.

[0009] In one embodiment of the present invention, a second control frequency value is generated in each cycle of the AC voltage.

[0010] In one embodiment of the present invention, the first control frequency value CF1 = [cf + K(df0) × df0] × F, where F is the instantaneous frequency value of the AC voltage, df0 ​​is equal to the passband frequency divided by the nominal frequency value of the AC voltage, cf is a positive frequency shift coefficient or a negative frequency shift coefficient, and K is a positive feedback gain.

[0011] In one embodiment of the present invention, K is linearly proportional to df0, and K is positive when df0 is equal to zero.

[0012] In one embodiment of the present invention, the second control frequency value CF2 = [cf' + K(df0) × df0] × F, where F is the instantaneous frequency value of the AC voltage, df0 ​​is equal to the passband frequency divided by the nominal frequency value of the AC voltage, cf' is the initial frequency coefficient, and K is the positive feedback gain.

[0013] In one embodiment of the present invention, the passband frequency is between 6 and 60 Hz.

[0014] In one embodiment of the present invention, an inverter device is also provided, coupled to a power grid and a load. The power grid generates an AC voltage. The inverter device includes a current-controlled inverter, a frequency detector, a processor, and a power control circuit. The current-controlled inverter receives a DC voltage, converts it into an AC current, and transmits the AC current to the power grid. The frequency detector is coupled to the power grid, the load, and the current-controlled inverter, and is used to detect the AC voltage and perform bandpass filtering on it to obtain the passband frequency of the AC voltage. The processor is coupled to the frequency detector and is used to receive the instantaneous frequency value and the passband frequency of the AC voltage. When the passband frequency is less than a critical frequency value, the processor generates a first control frequency value alternately based on a positive frequency shift coefficient and a negative frequency shift coefficient. The first control frequency value is associated with the instantaneous frequency value and the passband frequency of the AC voltage. When the passband frequency is not less than the critical frequency value, the processor continuously generates a second control frequency value based on an initial frequency shift coefficient, wherein the second control frequency value is associated with the instantaneous frequency value and the passband frequency of the AC voltage. The power control circuit is coupled to a current-controlled inverter, a load, a power grid, and a processor, and is used to receive one of a first control frequency value and a second control frequency value, AC current, AC voltage, a real power command, and a virtual power command, so as to adjust the frequency of the AC current accordingly.

[0015] In one embodiment of the present invention, the processor is used to generate a first control frequency value and a second control frequency value at different periods, wherein the period corresponding to the first control frequency value is greater than the period corresponding to the second control frequency value.

[0016] In one embodiment of the present invention, the processor is used to generate first control frequency values ​​corresponding to positive and negative frequency shift coefficients in the Nth and 2Nth cycles of the AC voltage, respectively, where N is a positive integer.

[0017] In one embodiment of the invention, the processor is used to generate a second control frequency value in each cycle of the AC voltage.

[0018] In one embodiment of the present invention, the first control frequency value CF1 = [cf + K(df0) × df0] × F, where F is the instantaneous frequency value of the AC voltage, df0 ​​is equal to the passband frequency divided by the nominal frequency value of the AC voltage, cf is a positive frequency shift coefficient or a negative frequency shift coefficient, and K is a positive feedback gain.

[0019] In one embodiment of the present invention, K is linearly proportional to df0, and K is a positive value when df0 is equal to zero.

[0020] In one embodiment of the present invention, the second control frequency value CF2 = [cf' + K(df0) × df0] × F, where F is the instantaneous frequency value of the AC voltage, df0 ​​is equal to the passband frequency divided by the nominal frequency value of the AC voltage, cf' is the initial frequency coefficient, and K is the positive feedback gain.

[0021] In one embodiment of the present invention, the passband frequency is between 6 and 60 Hz.

[0022] In one embodiment of the present invention, the frequency detector includes a voltage detector and a phase-locked loop (PLL). The voltage detector is coupled to the power grid, a processor, a load, and a current-controlled inverter, and is used to detect and output an AC voltage. The PLL is coupled to the voltage detector and the processor. The PLL is used to capture the AC voltage and perform bandpass filtering on it to obtain the passband frequency of the AC voltage.

[0023] In one embodiment of the present invention, the power control circuit includes an integrator, a power feedback circuit, a power controller, a current synchronization circuit, and a current controller. The integrator is coupled to a processor and is used to receive and integrate a first control frequency value or a second control frequency value to generate a phase angle. The power feedback circuit is coupled to a current-controlled inverter, a load, and the power grid, and is used to receive AC voltage and AC current, thereby generating real feedback power and virtual feedback power. The power controller is coupled to the power feedback circuit and is used to receive real feedback power, virtual feedback power, real power command, and virtual power command, thereby generating a control current. The current synchronization circuit is coupled to the power controller and the integrator, and is used to receive the control current and the phase angle, thereby generating a synchronization current. The current controller is coupled to the current synchronization circuit and the current-controlled inverter, and is used to receive the synchronization current to control the current-controlled inverter to adjust the frequency of the AC current.

[0024] Based on the above, the active islanding detection method generates a first control frequency value by alternating between positive and negative frequency shift coefficients when the passband frequency is less than the critical frequency value. This reduces the impact on the total harmonic distortion (THD) of the real / virtual power and output current of the inverter, thereby improving the control accuracy of THD and real / virtual power. When the passband frequency is not less than the critical frequency value, a second control frequency value is continuously generated based on the initial frequency shift coefficient, causing overvoltage (OV), undervoltage (UV), overfrequency (OF), or underfrequency (UF) phenomena in the power grid, thus accelerating islanding detection. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of an inverter device according to an embodiment of the present invention.

[0026] Figure 2 This is a schematic diagram of an active island detection method according to an embodiment of the present invention.

[0027] Figure 3 This is a graph of positive feedback gain versus df0 according to an embodiment of the present invention.

[0028] Figure reference numerals: 1-Inverter device; 10-Current-controlled inverter; 11-Frequency detector; 110-Voltage detector; 111-Phase-locked loop; 12-Processor; 13-Power control circuit; 130-Integrator; 131-Power feedback circuit; 132-Power controller; 133-Current synchronization circuit; 134-Current controller; 2-Load; 3-Switch; 4-Grid; VA-AC voltage; VD-DC voltage; IA-AC current; CF1-First control frequency value; CF2-Second control frequency value; PC-Real power command; QC-Virtual power command; Φ-Phase angle; P-Real feedback power; Q-Virtual feedback power; I-Control current; IS-Synchronization current; S10, S12, S14, S16, S18, S20, S22-Steps. Detailed Implementation

[0029] Embodiments of the present invention will be further explained below with reference to the accompanying drawings. Wherever possible, the same reference numerals represent the same or similar components in the drawings and description. In the drawings, shapes and thicknesses may be exaggerated for simplicity and convenience. It is understood that elements not specifically shown in the drawings or described in the description are forms known to those skilled in the art. Those skilled in the art can make various changes and modifications based on the content of this invention.

[0030] Unless otherwise specified, certain conditional clauses or words, such as "can," "could," "might," or "may," are generally intended to express features, elements, or steps that are present in the embodiments of this invention, but may also be interpreted as features, elements, or steps that may not be required. In other embodiments, these features, elements, or steps may be unnecessary.

[0031] In the following description of "one embodiment" or "an embodiment," the term refers to a specific element, structure, or feature associated with at least one embodiment. Therefore, the multiple descriptions of "one embodiment" or "an embodiment" appearing in various places below do not refer to the same embodiment. Furthermore, specific components, structures, and features in one or more embodiments may be combined in a suitable manner.

[0032] Certain terms are used in this specification and claims to refer to specific elements. However, those skilled in the art will understand that the same element may be referred to by different names. The specification and claims do not distinguish elements by differences in name, but rather by differences in function. The term "comprising" as used in this specification and claims is an open-ended term and should be interpreted as "comprising but not limited to." Furthermore, "coupled" here includes any direct and indirect connection means. Therefore, if the text describes a master element coupled to a slave element, it means that the master element can be directly connected to the slave element via electrical connection or signal connection methods such as wireless transmission or optical transmission, or indirectly electrically or signalally connected to the slave element via other elements or connection means.

[0033] This invention is described in particular by way of the following examples, which are merely illustrative. Various modifications and refinements can be made by those skilled in the art without departing from the spirit and scope of this disclosure. Therefore, the scope of protection of this invention is determined by the appended claims. Throughout the specification and claims, unless explicitly stated otherwise, the words “a” and “described” include “a or at least one” of the stated elements or components. Furthermore, as used herein, the singular article also includes descriptions of multiple elements or components unless clearly indicated from the specific context. Moreover, when applied in this description and all the following claims, unless explicitly stated otherwise, “in which” may include both “in which” and “therein”. The terms used throughout the specification and claims, unless otherwise specified, generally have their ordinary meaning in the context of this invention and its specific application. Certain terms used to describe the invention will be discussed below or elsewhere in this specification to provide additional guidance to practitioners in describing the invention. Examples throughout this specification, including examples of any terms discussed herein, are merely illustrative and do not limit the scope or meaning of the invention or any of the illustrative terms. Similarly, the invention is not limited to the various embodiments set forth in this specification.

[0034] The following presents an inverter device and its active islanding detection method according to the present invention. When the passband frequency is less than a critical frequency value, a first control frequency value is generated alternately based on positive and negative frequency shift coefficients to reduce the impact on the total harmonic distortion (THD) of the real / virtual power output and output current of the inverter device, thereby improving the control accuracy of THD and real / virtual power. When the passband frequency is not less than the critical frequency value, a second control frequency value is continuously generated based on the initial frequency shift coefficient, causing the power grid to experience overvoltage (OV), undervoltage (UV), overfrequency (OF), or underfrequency (UF) phenomena, thus accelerating islanding detection.

[0035] Figure 1 This is a schematic diagram of an inverter device according to an embodiment of the present invention. Please refer to... Figure 1 An inverter device 1 is coupled to a load 2 and connected to a power grid 4 via a switch 3. The power grid 4 generates an AC voltage VA. The inverter device 1 includes a current-controlled inverter 10, a frequency detector 11, a processor 12, and a power control circuit 13. In this embodiment, the switch 3 is in the ON state. The current-controlled inverter 10 receives a DC voltage VD and converts it into an AC current IA, which is then transmitted to the power grid 4 via the switch 3. The frequency detector 11 is coupled to the load 2 and the current-controlled inverter 10, and connected to the power grid 4 via the switch 3. The processor 12 is coupled to the frequency detector 11, and the power control circuit 13 is coupled to the current-controlled inverter 10, the load 2, and the processor 12, and connected to the power grid 4 via the switch 3.

[0036] Figure 2 This is a schematic diagram of an active island detection method according to an embodiment of the present invention. Figure 3 This is a graph showing the positive feedback gain versus df0 according to an embodiment of the present invention. Please refer to [link / reference]. Figure 1 , Figure 2 and Figure 3The active islanding detection method is described below. First, as shown in step S10, the frequency detector 11 detects the AC voltage VA and performs bandpass filtering on it to obtain the passband frequency BF of the AC voltage VA. As shown in step S12, the processor 12 receives the instantaneous frequency value of the AC voltage VA and the passband frequency BF. In a preferred embodiment, the passband frequency BF is between 6 and 60 Hz. Assuming that the fundamental frequency of the AC voltage VA is 40 Hz, and the harmonics of the AC voltage VA have 5 Hz and 80 Hz, then the passband frequency BF includes the fundamental frequency, and the harmonics will be filtered out. As shown in step S14, the processor 12 determines whether the passband frequency BF is less than a critical frequency value. If yes, steps S16 and S18 are performed sequentially; otherwise, steps S20 and S22 are performed sequentially. In step S16, the processor 12 generates a first control frequency value CF1 alternately based on positive and negative frequency shift coefficients, wherein the first control frequency value CF1 is associated with the instantaneous frequency value of the AC voltage VA and the passband frequency BF. The first control frequency value CF1 can reduce the impact on the total harmonic distortion (THD) of the real / virtual power and output current output by the inverter device 1, thereby improving the control accuracy of THD and real / virtual power. In step S18, the power control circuit 13 receives the first control frequency value CF1, the AC current IA, the AC voltage VA, a real power command PC, and a virtual power command QC, and adjusts the frequency of the AC current IA accordingly. In step S20, the processor 12 continuously generates a second control frequency value CF2 based on the initial frequency shift coefficient, wherein the second control frequency value CF2 is associated with the instantaneous frequency value of the AC voltage VA and the passband frequency BF. The second control frequency value CF2 can induce overvoltage (OV), undervoltage (UV), overfrequency (OF), or underfrequency (UF) phenomena in the power grid, thus accelerating islanding detection and smoothly allowing the mains power to disconnect and switch to standalone mode. In step S22, the power control circuit 13 receives the second control frequency value CF2, AC current IA, AC voltage VA, a real power command PC, and a virtual power command QC, and adjusts the frequency of the AC current IA accordingly. If substantially the same result can be obtained, these steps do not necessarily need to be followed. Figure 2 The execution order is as shown. In other words, by comparing the passband frequency BF with the critical frequency value, a distinction can be made between non-islanded and potentially islanded scenarios. In a non-islanded scenario, a first control frequency value CF1 is injected every N cycles, and the first control frequency value CF1 is generated alternately based on positive and negative frequency shift coefficients to minimize harmonics and power control errors. In a suspected islanded scenario, a fixed initial frequency shift coefficient cf' (i.e., the positive or negative frequency shift coefficient at the moment of detecting suspected islanding) is injected using a second control frequency value CF2, and the frequency shift coefficient is strengthened through positive feedback to achieve grid disconnection.

[0037] In some embodiments of the present invention, the processor 12 generates a first control frequency value CF1 and a second control frequency value CF2 at different periods, and the period corresponding to the first control frequency value CF1 is greater than the period corresponding to the second control frequency value CF2. For example, the processor 12 generates the first control frequency value CF1 corresponding to a positive frequency shift coefficient and a negative frequency shift coefficient in the Nth and 2Nth periods of the fundamental frequency of the AC voltage VA, respectively, where N is a positive integer. When N is greater than or equal to 2, the first control frequency value CF1 will not be generated in any period other than the Nth and 2Nth periods of the fundamental frequency of the AC voltage VA. The processor 12 generates the second control frequency value CF2 in each period of the fundamental frequency of the AC voltage VA. In some embodiments, the first control frequency value CF1 = [cf + K(df0) × df0] × F, where df0 is equal to the passband frequency divided by the nominal frequency value of the AC voltage, cf is the positive or negative frequency shift coefficient, and K is the positive feedback gain. The second control frequency value CF2 = [cf' + K(df0) × df0] × F, where cf' is the initial frequency coefficient. To enhance the positive feedback effect and accelerate the tripping protection, K is linearly proportional to df0. When df0 equals zero, K is a positive value, such as... Figure 3 As shown.

[0038] In some embodiments, the frequency detector 11 may include, but is not limited to, a voltage detector 110 and a phase-locked loop 111. The voltage detector 110 is coupled to the processor 12, the load 2, and the current-controlled inverter 10, and is coupled to the power grid 4 via a switch 3. The phase-locked loop 111 is coupled to the voltage detector 110 and the processor 12. The voltage detector 110 detects and outputs an AC voltage VA. The phase-locked loop 111 captures the AC voltage VA and performs a bandpass filter on it to obtain the passband frequency BF of the AC voltage VA.

[0039] In some embodiments, the power control circuit 13 may include, but is not limited to, an integrator 130, a power feedback circuit 131, a power controller 132, a current synchronization circuit 133, and a current controller 134. The integrator 130 is coupled to the processor 12, the power feedback circuit 131 is coupled to the current-controlled inverter 10 and the load 2, and is coupled to the power grid 4 via a switch 3. The power controller 132 is coupled to the power feedback circuit 131, the current synchronization circuit 133 is coupled to the power controller 132 and the integrator 130, and the current controller 134 is coupled to the current synchronization circuit 133 and the current-controlled inverter 10. The integrator 130 receives and integrates a first control frequency value CF1 or a second control frequency value CF2 to generate a phase angle Φ. The power feedback circuit 131 receives an AC voltage VA and an AC current IA, and generates real feedback power P and virtual feedback power Q accordingly. The power feedback circuit 131 can measure the actual generated real power and virtual power. Power controller 132 receives real feedback power P, virtual feedback power Q, real power command PC, and virtual power command QC, and generates a control current I accordingly. Power controller 132 can output a certain amount of real power and virtual power based on the real power command PC and virtual power command QC. Current synchronization circuit 133 receives the control current I and phase angle Φ, and generates a synchronization current IS accordingly. For example, IS = I × cosΦ. Current controller 134 receives the synchronization current IS and uses it to control the frequency of the AC current IA of the current-controlled inverter 10.

[0040] According to the above embodiments, the active islanding detection method achieves the effect of actively detecting unplanned islanding systems while minimizing the injected disturbance, smoothly allowing the mains power to break away and switch to an independent operating mode, and can improve the control accuracy of real / virtual power under grid-connected conditions, which complies with grid-connected regulations.

[0041] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of the present invention. Therefore, all equivalent variations and modifications made in accordance with the shape, structure, features and spirit described in the claims of the present invention should be included within the scope of the present invention.

Claims

1. An active islanding detection method applied to an inverter device coupled to a power grid and a load, the power grid generating an AC voltage, the inverter device receiving a DC voltage, converting it into an AC current, and transmitting the AC current to the power grid, characterized in that, The active island detection method includes the following steps: The AC voltage is bandpass filtered to obtain the passband frequency of the AC voltage; Determine whether the passband frequency is less than a critical frequency value: If so, a first control frequency value is generated by alternating positive and negative frequency shift coefficients, wherein the first control frequency value is associated with the instantaneous frequency value of the AC voltage and the passband frequency; as well as If not, a second control frequency value is continuously generated based on the initial frequency shift coefficient, wherein the second control frequency value is associated with the instantaneous frequency value of the AC voltage and the passband frequency; and The frequency of the AC current is adjusted based on one of the first control frequency value and the second control frequency value, the AC current, the AC voltage, a real power command, and a virtual power command.

2. The active island detection method as described in claim 1, characterized in that, The first control frequency value and the second control frequency value are generated with different periods, and the period corresponding to the first control frequency value is greater than the period corresponding to the second control frequency value.

3. The active island detection method as described in claim 1, characterized in that, The first control frequency values ​​corresponding to the positive frequency shift coefficient and the negative frequency shift coefficient are generated in the Nth and 2Nth cycles of the AC voltage, respectively, where N is a positive integer.

4. The active island detection method as described in claim 1, characterized in that, The second control frequency value is generated in each cycle of the AC voltage.

5. The active island detection method as described in claim 1, characterized in that, The first control frequency value CF1 = [cf + K(df0) × df0] × F, where F is the instantaneous frequency value of the AC voltage, df0 ​​is equal to the passband frequency divided by the nominal frequency value of the AC voltage, cf is the positive frequency shift coefficient or the negative frequency shift coefficient, and K is the positive feedback gain.

6. The active island detection method as described in claim 5, characterized in that, K is linearly proportional to df0, and K is positive when df0 is zero.

7. The active island detection method as described in claim 1, characterized in that, The second control frequency value CF2 = [cf' + K(df0) × df0] × F, where F is the instantaneous frequency value of the AC voltage, df0 ​​is equal to the passband frequency divided by the nominal frequency value of the AC voltage, cf' is the initial frequency coefficient, and K is the positive feedback gain.

8. The active island detection method as described in claim 7, characterized in that, K is linearly proportional to df0, and K is positive when df0 is zero.

9. The active island detection method as described in claim 1, characterized in that, The passband frequency is between 6 and 60 Hz.

10. An inverter device coupled to a power grid and a load, wherein the power grid generates an alternating current voltage, characterized in that, The inverter device includes: A current-controlled inverter is used to receive a DC voltage, convert it into an AC current, and transmit the AC current to the power grid. A frequency detector is coupled to the power grid, the load and the current-controlled inverter, and is used to detect the AC voltage and perform bandpass filtering on it to obtain the passband frequency of the AC voltage. A processor, coupled to the frequency detector, is configured to receive an instantaneous frequency value of the AC voltage and the passband frequency, wherein when the passband frequency is less than a critical frequency value, the processor alternately generates a first control frequency value based on positive and negative frequency shift coefficients, wherein the first control frequency value is associated with the instantaneous frequency value of the AC voltage and the passband frequency; and when the passband frequency is not less than the critical frequency value, the processor continuously generates a second control frequency value based on an initial frequency shift coefficient, wherein the second control frequency value is associated with the instantaneous frequency value of the AC voltage and the passband frequency; and A power control circuit is coupled to the current-controlled inverter, the load, the power grid and the processor, and is used to receive one of the first control frequency value and the second control frequency value, the AC current, the AC voltage, a real power command and a virtual power command, so as to adjust the frequency of the AC current accordingly.

11. The inverter device as claimed in claim 10, characterized in that, The processor is configured to generate the first control frequency value and the second control frequency value at different periods, wherein the period corresponding to the first control frequency value is greater than the period corresponding to the second control frequency value.

12. The inverter device as claimed in claim 10, characterized in that, The processor is used to generate the first control frequency value corresponding to the positive frequency shift coefficient and the negative frequency shift coefficient in the Nth and 2Nth cycles of the AC voltage, respectively, where N is a positive integer.

13. The inverter device as claimed in claim 10, characterized in that, The processor is used to generate the second control frequency value in each cycle of the AC voltage.

14. The inverter device as claimed in claim 10, characterized in that, The first control frequency value CF1 = [cf + K(df0) × df0] × F, where F is the instantaneous frequency value of the AC voltage, df0 ​​is equal to the passband frequency divided by the nominal frequency value of the AC voltage, cf is the positive frequency shift coefficient or the negative frequency shift coefficient, and K is the positive feedback gain.

15. The inverter device as claimed in claim 14, characterized in that, K is linearly proportional to df0, and K is positive when df0 is zero.

16. The inverter device as claimed in claim 10, characterized in that, The second control frequency value CF2 = [cf' + K(df0) × df0] × F, where F is the instantaneous frequency value of the AC voltage, df0 ​​is equal to the passband frequency divided by the nominal frequency value of the AC voltage, cf' is the initial frequency coefficient, and K is the positive feedback gain.

17. The inverter device as claimed in claim 16, characterized in that, K is linearly proportional to df0, and K is positive when df0 is zero.

18. The inverter device as claimed in claim 10, characterized in that, The passband frequency is between 6 and 60 Hz.

19. The inverter device as claimed in claim 10, characterized in that, The frequency detector includes: A voltage detector, coupled to the power grid, the processor, the load, and the current-controlled inverter, is used to detect the AC voltage and output the AC voltage; and A phase-locked loop is coupled to the voltage detector and the processor, wherein the phase-locked loop is used to capture the AC voltage and perform bandpass filtering on it to obtain the passband frequency of the AC voltage.

20. The inverter device as claimed in claim 10, characterized in that, The power control circuit includes: An integrator, coupled to the processor, is used to receive and integrate the first control frequency value or the second control frequency value to generate a phase angle; A power feedback circuit is coupled to the current-controlled inverter, the load and the power grid, and is used to receive the AC voltage and the AC current, and thereby generate real feedback power and virtual feedback power. A power controller is coupled to the power feedback circuit and is used to receive the real feedback power, the virtual feedback power, the real power command and the virtual power command, and generate a control current accordingly. A current synchronization circuit, coupled to the power controller and the integrator, is used to receive the control current and the phase angle, and thereby generate a synchronization current; and A current controller is coupled to the current synchronization circuit and the current-controlled inverter, and is used to receive the synchronization current so as to control the current-controlled inverter to adjust the frequency of the AC current.

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