Method, device and grid-connected converter for reducing output current harmonic distortion rate

Abstract

The present invention discloses a method, device and grid-connected converter for reducing the harmonic distortion rate of output current. The DC side of the grid-connected converter includes a voltage outer loop and a current inner loop. The method includes: obtaining an output signal of a PI controller provided in the voltage outer loop; filtering out the harmonic components in the output signal of the PI controller to obtain a reference signal of the current inner loop; inputting the reference signal of the current inner loop into the current inner loop to obtain the output current. The output current harmonic distortion rate of the grid-connected converter can be effectively reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of grid-connected converters, and in particular to a method and device for reducing the harmonic distortion rate of an output current, and a grid-connected converter. Background Art

[0002] The grid-connected converter is an important component of the distributed energy system and an important interface between the power grid and photovoltaic and energy storage devices. When the grid-connected converter works normally, there are two main working modes, namely off-grid and grid-connected mode. In the off-grid mode, the power grid neither generates electricity nor absorbs energy. At this time, the converter is in a disconnected state. In the grid-connected mode, the photovoltaic energy storage system can be divided into two conditions: power consumption and grid feeding. In the grid feeding condition, the photovoltaic device or energy storage device feeds back energy to the grid through the converter. In the power consumption condition, the grid supplies power to the energy storage device through the converter. In this process, the DC side capacitor absorbs reactive power while transmitting active power. The reactive power absorbed by the capacitor will cause the capacitor voltage to generate ripple components, among which the ripple content of the double frequency is relatively large. In the grid-connected mode, the control strategy of the DC voltage outer loop and the inductor current inner loop is usually adopted. It is difficult for the voltage outer loop to suppress the ripple component of the double frequency. Therefore, the output signal of the voltage outer loop will also contain a large number of double frequency harmonic components. Because its output signal is the reference given signal of the current inner loop, the ripple component of the capacitor voltage will affect the output of the current inner loop and cause the output current harmonic distortion rate to increase. Summary of the invention

[0003] The embodiments of the present invention provide a method, a device and a grid-connected converter for reducing the harmonic distortion rate of an output current, so as to solve the problem of high output current distortion rate of the grid-connected converter in the prior art.

[0004] To solve the above technical problems, the present invention provides a method for reducing the harmonic distortion rate of output current, which is applied to a grid-connected switch. The DC side of the grid-connected switch includes a voltage outer loop and a current inner loop. The method includes: obtaining an output signal of a PI controller arranged in the voltage outer loop; filtering out the harmonic components in the output signal of the PI controller to obtain a reference signal of the current inner loop; and inputting the reference signal of the current inner loop into the current inner loop to obtain the output current.

[0005] Optionally, the harmonic components in the output signal of the PI controller are filtered out to obtain a reference signal of the current inner loop, including: filtering out the harmonic components in the output signal of the PI controller by a cancellation filtering method; performing phase compensation on the phase delay generated during the cancellation filtering process to obtain a reference signal of the current inner loop.

[0006] Optionally, the harmonic component includes a double frequency harmonic component of the capacitor voltage on the DC side of the grid-connected converter; or, also includes other DC harmonic components.

[0007] Optionally, according to the above method, after the output current is obtained, it includes: modulating it by SVPWM (space vector pulse width modulation) and then outputting it through a driving circuit.

[0008] Optionally, filtering out harmonic components in the output signal of the PI controller by a cancellation filtering method includes: performing cancellation filtering according to the following harmonic cancellation operator:

[0009]

[0010] Where N is the filter order; z is a complex variable in the Z domain; r is the pole radius, and 0 <r<1。

[0011] Optionally, performing phase compensation on the phase delay generated in the above-mentioned cancellation and filtering process to obtain a reference signal of the current inner loop includes: performing phase compensation according to the following compensation operator:

[0012]

[0013] Where N is the filter order; z is a complex variable in the Z domain; r is the pole radius, and 0 <r<1。

[0014] On the other hand, the present invention also provides a device for reducing the harmonic distortion rate of the output current, which is applied to a grid-connected switch. The DC side of the grid-connected switch includes a voltage outer loop and a current inner loop. The device includes: an acquisition module, which acquires the output signal of a PI controller arranged in the voltage outer loop; a filtering module, which filters out the harmonic components in the output signal of the PI controller to obtain a reference signal of the current inner loop; and an input module, which inputs the reference signal of the current inner loop into the current inner loop to obtain the output current.

[0015] Optionally, the filtering module includes: a harmonic cancellation unit, which filters out the harmonic components in the output signal of the PI controller by a cancellation filtering method; and a compensation unit, which performs phase compensation on the phase delay generated in the cancellation filtering process to obtain a reference signal of the current inner loop.

[0016] Optionally, the harmonic cancellation operator of the harmonic cancellation unit is:

[0017]

[0018] Where N is the filter order; z is a complex variable in the Z domain; r is the pole radius, and 0 <r<1。

[0019] Optionally, phase compensation is performed according to the following compensation operator:

[0020]

[0021] Where N is the filter order; z is a complex variable in the Z domain; r is the pole radius, and 0 <r<1。

[0022] On the other hand, the present invention further provides a grid-connected switch, comprising the above-mentioned device for reducing the harmonic distortion rate of output current.

[0023] By applying the technical solution of the present invention, the output signal after passing through the PI controller is filtered to eliminate the harmonic components in the current inner loop reference signal, thereby reducing the harmonic distortion rate of the output current of the current inner loop. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a structural block diagram of a grid-connected converter in the prior art;

[0025] Figure 2 It is a control block diagram of a converter in grid-connected mode in the prior art;

[0026] Figure 3 is a flow chart of a method for reducing the harmonic distortion rate of output current of a grid-connected converter according to an embodiment of the present invention;

[0027] Figure 4 is a control block diagram of a converter in a grid-connected mode according to an embodiment of the present invention;

[0028] Figure 5 is a structural block diagram of a device for reducing the harmonic distortion rate of output current of a grid-connected converter according to an embodiment of the present invention;

[0029] Figure 6 is an amplitude-frequency characteristic curve of a harmonic cancellation operator according to an embodiment of the present invention;

[0030] Figure 7 is G according to an embodiment of the present invention h (z)G c (z) Amplitude-frequency characteristic curve.

[0031] Reference numerals: 100 - acquisition module 200 - filtering module 300 - input module DETAILED DESCRIPTION

[0032] In order to make the purpose, technical scheme and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0033] The terms used in the embodiments of the present invention are only for the purpose of describing specific embodiments, and are not intended to limit the present invention. The singular forms "a", "said" and "the" used in the embodiments of the present invention and the appended claims are also intended to include plural forms, unless the context clearly indicates other meanings, and "multiple" generally includes at least two.

[0034] It should be understood that the term "and / or" used in this article is only a description of the association relationship of associated objects, indicating that there can be three relationships. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character " / " in this article generally indicates that the associated objects before and after are in an "or" relationship.

[0035] As used herein, the words "if" and "if" may be interpreted as "at the time of" or "when" or "in response to determining" or "in response to detecting", depending on the context. Similarly, the phrases "if it is determined" or "if (stated condition or event) is detected" may be interpreted as "when it is determined" or "in response to determining" or "when detecting (stated condition or event)" or "in response to detecting (stated condition or event)", depending on the context.

[0036] It should also be noted that the term "includes", "comprising" or any other variation thereof is intended to cover non-exclusive inclusion, so that a commodity or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such commodity or device. In the absence of more restrictions, the elements defined by the sentence "comprising a ..." do not exclude the existence of other identical elements in the commodity or device including the elements.

[0037] The structural block diagram of the grid-connected converter in the prior art is as follows: Figure 1 As shown, it is an important component of the distributed energy system and an important interface between the power grid and photovoltaic and energy storage equipment. Figure 1 The grid-connected converter shown has the functions of rectification and reverse bidirectional conversion. Its DC side is composed of a single or multiple capacitors, and the voltage on the capacitor is the DC bus voltage. When the grid-connected converter works normally, there are mainly two working modes, namely off-grid mode and grid-connected mode.

[0038] In off-grid mode, the grid neither generates electricity nor absorbs energy. At this time, the grid-connected converter is in a disconnected state, and the photovoltaic device or energy storage device feeds back energy to the grid through the grid-connected converter.

[0039] like Figure 2 As shown, in the grid-connected mode, U dc is the capacitor voltage on the DC side of the converter, i gabcis the grid-side inductor current of the grid-connected converter. The voltage outer loop uses a PI controller to track the voltage reference instruction, and the current inner loop also uses a PI controller to track the output signal of the voltage outer loop. In the grid-connected mode, the fundamental component of the grid-side output current of the converter is almost in phase with the grid voltage, so the output power of the grid will contain pulsating components of double frequency and other frequencies. According to the power balance (energy storage only absorbs active power), this double frequency and other frequency pulsating power is absorbed by the DC side capacitor. Therefore, the capacitor voltage will contain double frequency harmonic components and other harmonic components, where the amplitude of the double frequency component is inversely proportional to the capacitance of the capacitor. Considering the hardware cost, filtering capability and volume of the DC side capacitor, the capacitance value is usually within the range that makes the capacitor voltage ripple content less than 10%.

[0040] Recombination Figure 2 It can be seen that when the capacitor voltage contains a harmonic component of double frequency, it is difficult for the PI controller to suppress the double frequency component. Then the output signal of the voltage outer loop will contain a harmonic component of double frequency, that is, the reference signal of the current inner loop contains a harmonic component of double frequency. For the current inner loop, the harmonic component in the reference signal will cause the grid-side output current of the grid-connected converter to contain harmonic components, which deteriorates the waveform quality of the current. Therefore, the double frequency component in the capacitor voltage will eventually deteriorate the waveform quality of the grid-side current of the grid-connected converter, resulting in an increase in the harmonic distortion rate of the output current.

[0041] To solve the above problems, the present invention provides a method for reducing the harmonic distortion rate of the output current of a grid-connected converter, which is applied to a grid-connected switch. The DC side of the grid-connected switch includes a voltage outer loop and a current inner loop. The method includes: obtaining an output signal of a PI controller arranged in the voltage outer loop; filtering out the harmonic components in the output signal of the PI controller to obtain a reference signal of the current inner loop; and inputting the reference signal of the current inner loop into the current inner loop to obtain the output current.

[0042] The optional embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0043] Figure 3 is a flow chart of a method for reducing the harmonic distortion rate of the output current of a grid-connected converter according to an embodiment of the present invention, such as Figure 3 As shown, the method comprises the following steps:

[0044] Step S101, obtaining an output signal of a PI controller provided in a voltage outer loop;

[0045] Step S102, filtering out the harmonic components in the output signal of the PI controller to obtain a reference signal of the current inner loop;

[0046] Step S103: input the reference signal of the current inner loop into the current inner loop to obtain the output current.

[0047] By filtering out the harmonic components in the output signal of the PI controller, a current inner-loop reference signal without harmonic components can be obtained, thereby eliminating the harmonic distortion rate of the output current of the current inner loop.

[0048] As a possible implementation, filtering out the harmonic components in the output signal of the PI controller to obtain the reference signal of the current inner loop includes: filtering out the harmonic components in the output signal of the PI controller by means of cancellation filtering; compensating the phase delay generated in the above cancellation filtering process to obtain the reference signal of the current inner loop.

[0049] Figure 4 It is the control block diagram of the converter in the grid-connected mode according to the embodiment of the present invention, as Figure 4 shown. First, control the output signal to pass through the harmonic cancellation module to cancel or filter out the harmonic components in the output signal of the PI controller. However, the harmonic cancellation module will generate a phase delay, reducing the dynamic performance of the voltage outer loop. Therefore, it is necessary to compensate the phase delay generated after harmonic cancellation through a compensator to improve the dynamic performance of the voltage outer loop. Therefore, this method can not only filter out the harmonic components in the capacitor voltage, but also ensure good dynamic performance of the voltage outer loop.

[0050] As a possible implementation, the harmonic components include the second-harmonic components of the capacitor voltage on the DC side in the grid-connected converter; or, other DC harmonic components are also included.

[0051] This method mainly filters out or cancels the second-harmonic components, and can also filter out other DC harmonic components.

[0052] As a possible implementation, after obtaining the output current according to the above method, it includes: outputting through the drive circuit after being modulated by SVPWM (Space Vector Pulse Width Modulation).

[0053] Although there are multiple switchings in each small interval of SVPWM, each switch switching only involves one device, so the switching loss is small. Using the voltage space vector to directly generate the three-phase PWM wave, the calculation is simple. The maximum value of the fundamental wave of the line voltage output by the inverter is the DC side voltage, which is 15% higher than the output voltage of the general SPWM inverter.

[0054] As a possible implementation, filtering out the harmonic components in the output signal of the PI controller by means of cancellation filtering includes: performing cancellation filtering according to the following harmonic cancellation operator:

[0055]

[0056] where N is the filtering order; z is the complex variable in the Z domain; r is the pole radius, and 0 < r < 1. N in this formula is a positive integer.

[0057] As a possible implementation, phase compensation is performed on the phase delay generated during the cancellation filtering process to obtain a reference signal for the inner current loop, including: performing phase compensation according to the following compensation operator:

[0058]

[0059] where N is the filtering order; z is the complex variable in the Z domain; r is the pole radius, and 0 < r < 1. N in this formula is a positive integer.

[0060] The above technical solution has the following technical effects: By performing a filtering operation on the output signal after passing through the PI controller, the harmonic components in the reference signal of the inner current loop are eliminated, thereby reducing the harmonic distortion rate of the output current of the inner current loop, and the phase delay is eliminated during this process.

[0061] Corresponding to Figure 1 the flowchart of the method for reducing the harmonic distortion rate of the output current of the grid-connected converter introduced, this embodiment provides a device for reducing the harmonic distortion rate of the output current of the grid-connected converter, as Figure 5 shown in the structural block diagram of the device for reducing the harmonic distortion rate of the output current of the grid-connected converter, the device includes:

[0062] An acquisition module 100 that acquires the output signal of the PI controller provided in the outer voltage loop;

[0063] A filtering module 200 that filters out the harmonic components in the output signal of the PI controller to obtain a reference signal for the inner current loop;

[0064] An input module 300 that inputs the reference signal of the inner current loop into the inner current loop to obtain an output current.

[0065] By filtering out the harmonic components in the output signal of the PI controller through the filtering module, a reference signal for the inner current loop without harmonic components can be obtained, thereby eliminating the harmonic distortion rate of the output current of the inner current loop.

[0066] As a possible implementation, the filtering module 200 includes:

[0067] A harmonic cancellation unit that filters out the harmonic components in the output signal of the PI controller by the cancellation filtering method;

[0068] A compensation unit that performs phase compensation on the phase delay generated during the above cancellation filtering process to obtain a reference signal for the inner current loop.

[0069] Figure 4 is the control block diagram of the converter under the grid-connected mode according to the embodiment of the present invention, as Figure 4As shown, first, control the output signal to pass through the harmonic cancellation module to cancel or filter out the harmonic components in the output signal of the PI controller. However, the harmonic cancellation module will introduce phase delay, reducing the dynamic performance of the voltage outer loop. Therefore, it is necessary to use a compensator to compensate for the phase delay generated after harmonic cancellation to improve the dynamic performance of the voltage outer loop. Therefore, this method can not only filter out the harmonic components in the capacitor voltage but also ensure good dynamic performance of the voltage outer loop.

[0070] As a possible implementation, the harmonic cancellation operator of the harmonic cancellation unit is:

[0071]

[0072] where N is the filtering order; z is the complex variable in the Z domain; r is the pole radius, and 0 < r < 1. N in this formula is a positive integer.

[0073] As a possible implementation, perform phase compensation according to the following compensation operator:

[0074]

[0075] where N is the filtering order; z is the complex variable in the Z domain; r is the pole radius, and 0 < r < 1. N in this formula is a positive integer.

[0076] On the other hand, an embodiment of the present invention also provides a grid-connected exchanger, including the device for reducing the harmonic distortion rate of the output current described above.

[0077] The following is illustrated with a specific embodiment: In the discrete process, the sampling frequency is set to 9.6 kHz, and the fundamental frequency is 50 Hz. When N = 96 and r = 0.9996, the amplitude-frequency characteristic of the harmonic cancellation operator G h (z) is as Figure 6 shown. As can be Figure 6 seen, all even harmonic components are completely filtered out, including the DC component (which needs to be retained). In addition, the phase in the lower frequency band (less than 100 Hz) also changes. Therefore, it is necessary to compensate for the amplitude of the DC component and the phase in the lower frequency band. Compensate according to the compensation operator. After compensation, the amplitude-frequency characteristic of G h (z)G c (z) is as Figure 7 shown. By comparing Figure 6 it can be found that the amplitude of the DC component is compensated, and the phase in the lower frequency band is also compensated, and the phase is close to 0. Therefore, the harmonic cancellation unit and the compensation unit can not only filter out harmonic components but also have very little phase delay in the low-frequency band.

[0078] The present invention is described by taking a photovoltaic storage system under a three-phase grid as an example, and can also be applied to a photovoltaic storage system under a single-phase grid. The harmonic cancellation module and compensator are also applicable to other dual-loop or multi-loop controllers.

[0079] The above technical solution has the following technical effects: by filtering the output signal after passing through the PI controller, the harmonic components in the current inner loop reference signal are eliminated, thereby reducing the harmonic distortion rate of the current inner loop output current, and eliminating phase delay in the process.

[0080] The above product can execute the method provided by the embodiment of the present invention, and has the functional modules and beneficial effects corresponding to the execution method. For technical details not described in detail in this embodiment, please refer to the method provided by the embodiment of the present invention.

[0081] The device embodiments described above are merely illustrative, wherein the units described as separate components may or may not be physically separated, and the components shown as units may or may not be physical units, that is, they may be located in one place or distributed on multiple network units. Some or all of the modules may be selected according to actual needs to achieve the purpose of the solution of this embodiment.

[0082] Through the description of the above implementation methods, those skilled in the art can clearly understand that each implementation method can be implemented by means of software plus a necessary general hardware platform, and of course, it can also be implemented by hardware. Based on this understanding, the above technical solution is essentially or the part that contributes to the prior art can be embodied in the form of a software product, and the computer software product can be stored in a computer-readable storage medium, such as ROM / RAM, a disk, an optical disk, etc., including a number of instructions for a computer device (which can be a personal computer, a server, or a network device, etc.) to execute the methods described in each embodiment or some parts of the embodiments.

[0083] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for reducing the harmonic distortion rate of output current, applied to a grid-connected switch, wherein the DC side of the grid-connected switch includes a voltage outer loop and a current inner loop, characterized in that: The method includes: Obtaining an output signal of a PI controller provided in a voltage outer loop; The harmonic components in the output signal of the PI controller are filtered out to obtain the reference signal of the current inner loop; the method includes: filtering out the harmonic components in the output signal of the PI controller by a cancellation filtering method; performing phase compensation on the phase delay generated in the cancellation filtering process to obtain the reference signal of the current inner loop; Inputting the reference signal of the current inner loop into the current inner loop to obtain an output current; The method of filtering out the harmonic components in the output signal of the PI controller by cancellation filtering includes: performing cancellation filtering according to the following harmonic cancellation operator: Where N is the filter order; z is a complex variable in the Z domain; r is the pole radius, and 0 <r<1。 2. The method according to claim 1, characterized in that The harmonic component includes a double frequency harmonic component of the capacitor voltage on the DC side of the grid-connected converter; or, further includes other DC harmonic components.

3. The method according to any one of claims 1 to 2, characterized in that: After the output current is obtained, the method includes: After SVPWM space vector pulse width modulation, it is output through the drive circuit.

4. The method according to claim 1, characterized in that: The phase delay generated in the cancellation filtering process is phase compensated to obtain a reference signal of the current inner loop, including: Phase compensation is performed according to the following compensation operator: Where N is the filter order; z is a complex variable in the Z domain; r is the pole radius, and 0 <r<1。 5. A device for reducing the harmonic distortion rate of output current, applied to a grid-connected switch, for implementing the method for reducing the harmonic distortion rate of output current according to any one of claims 1 to 4, wherein the DC side of the grid-connected switch comprises a voltage outer loop and a current inner loop, and is characterized in that: The device includes: An acquisition module is used to acquire an output signal of a PI controller provided in a voltage outer loop; A filtering module, for filtering out harmonic components in the output signal of the PI controller to obtain a reference signal of the current inner loop; An input module inputs the reference signal of the current inner loop into the current inner loop to obtain an output current.

6. The device according to claim 5, characterized in that The filtering module comprises: A harmonic cancellation unit, which filters out harmonic components in the output signal of the PI controller by a cancellation filtering method; The compensation unit performs phase compensation on the phase delay generated in the cancellation filtering process to obtain a reference signal of the current inner loop.

7. The device according to claim 6, characterized in that: The harmonic cancellation operator of the harmonic cancellation unit is: Where N is the filter order; z is a complex variable in the Z domain; r is the pole radius, and 0 <r<1。 8. The device according to claim 6, characterized in that: Phase compensation is performed according to the following compensation operator: Where N is the filter order; z is a complex variable in the Z domain; r is the pole radius, and 0 <r<1。 9. A grid-connected switch, characterized in that: The grid-connected converter comprises the device for reducing the harmonic distortion rate of the output current according to any one of claims 5 to 8.

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