Filter circuit, filter control device and frequency converter system

Abstract

The utility model provides a filter circuit, a filter control device and a frequency converter system. The filter circuit comprises a first inductor device, a first capacitor device and a plurality of filter branches; the first end of the first capacitor device is connected with the first inductor device, and the second end of the first capacitor device is connected with the plurality of filter branches; the plurality of filtering branches are connected in parallel; at least part of the filtering branches comprise resonance parameter adjusting devices, and the resonance parameter adjusting devices in different filtering branches are different; the filtering branch also comprises an on-off control device which is used for controlling the on-off of the filtering branch and the first inductance device. According to the mode, different resonance parameter adjusting devices can be communicated, so that frequency band harmonic waves existing in a current loop are filtered out, the harmonic waves of all frequency bands can be effectively filtered out by connecting the different resonance parameter adjusting devices, a wide-frequency-band filtering function is achieved, and the harmonic wave filtering effect of all the frequency bands is improved.

Description

Technical Field

[0001] The present application relates to the field of circuit technologies, and in particular, to a filtering circuit, a filtering control device, and an inverter system. Background Art

[0002] An inductor and a filtering capacitor are usually arranged between the grid side of a converter and the power grid. The inductor, the filtering capacitor, and the equivalent leakage inductance of the box-type transformer on the grid side form an LCL filtering topology. Since the filtering capacitor presents a low impedance characteristic to high-frequency harmonics and can absorb high-frequency harmonics well, the LCL filtering topology can effectively filter high-frequency harmonics, but has a poor suppression effect on low-frequency harmonics. Summary of the Utility Model

[0003] In view of this, embodiments of the present specification provide a filtering circuit, a filtering control device, and an inverter system to effectively filter harmonics in each frequency band, have a wide-band filtering function, and improve the harmonic filtering effect in each frequency band.

[0004] In a first aspect, embodiments of the present specification provide a filtering circuit. The filtering circuit includes a first inductor device, a first capacitor device, and a plurality of filtering branches; a first end of the first capacitor device is connected to the first inductor device, and a second end of the first capacitor device is connected to the plurality of filtering branches; the plurality of filtering branches are connected in parallel; at least some of the filtering branches include resonance parameter adjustment devices, and the resonance parameter adjustment devices in different filtering branches are different; the filtering branches further include on-off control devices for controlling the on-off between the filtering branches and the first inductor device.

[0005] The above resonance parameter adjustment device includes a resonance peak adjustment device and / or a resonance frequency point adjustment device.

[0006] If a filtering branch includes a resonance parameter adjustment device, the resonance parameter adjustment device in the filtering branch includes: a resistor device for reducing the resonance peak; or, a second inductor device for reducing the resonance frequency point; or, a second capacitor device for increasing the resonance frequency point.

[0007] The above filtering branches include multiple types of the following: a first branch, the first branch includes a first on-off control device and a resistor device; the first on-off control device and the resistor device are connected in series; a second branch, the second branch includes a second on-off control device and a second inductor device; the second on-off control device and the second inductor device are connected in series; a third branch, the third branch includes a third on-off control device and a second capacitor device; the third on-off control device and the second capacitor device are connected in series; a fourth branch, the fourth branch includes a fourth on-off control device.

[0008] The above first capacitor device includes an AC capacitor.

[0009] In a second aspect, an embodiment of this specification provides a filtering control device. The filtering control device includes a filtering circuit and further includes a control device; the control device is used to connect to the power grid and detect the harmonics of the power grid, and the control device is connected to the filtering circuit and controls the on / off of the filtering branches in the filtering circuit.

[0010] The above-mentioned control device is connected to the on / off control devices in each filtering branch of the filtering circuit and is used to control the on / off of the on / off control devices.

[0011] In a third aspect, an embodiment of this specification provides a frequency converter system. The frequency converter system includes a frequency converter and a filtering control device; the filtering control device is arranged between the frequency converter and the power grid.

[0012] The first inductor device in the above-mentioned filtering control device is connected to the frequency converter, and the first capacitor device and the control device in the filtering control device are respectively connected to the power grid.

[0013] The above-mentioned frequency converter includes a full-power converter or a doubly-fed converter.

[0014] The above-mentioned frequency converter includes a doubly-fed converter, and the first capacitor device in the filtering control device is further connected to a generator; the generator is also connected to the doubly-fed converter.

[0015] The above-mentioned filtering circuit includes a first inductor device, a first capacitor device, and a plurality of filtering branches; the first end of the first capacitor device is connected to the first inductor device, the second end of the first capacitor device is connected to the plurality of filtering branches; the plurality of filtering branches are connected in parallel; at least some of the filtering branches include resonance parameter adjustment devices, and the resonance parameter adjustment devices in different filtering branches are different; the filtering branches further include on / off control devices for controlling the on / off of the filtering branches and the first inductor device.

[0016] A plurality of parallel filtering branches are arranged in the filtering circuit, and different resonance parameter adjustment devices are arranged in different filtering branches; by controlling the on / off of the on / off control devices in the filtering branches, different resonance parameter adjustment devices can be connected, so as to filter out the harmonic waves in the current loop. By accessing different resonance parameter adjustment devices, the harmonic waves in each frequency band can be effectively filtered out, having a wide-band filtering function and improving the harmonic filtering effect in each frequency band.

[0017] To make the above-mentioned objects, features, and advantages of this application more obvious and understandable, the following specifically gives preferred embodiments and, in conjunction with the accompanying drawings, the detailed description is as follows. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] To more clearly illustrate the specific embodiments of the present application or the technical solutions in the prior art, the following will briefly introduce the drawings required for the description of the specific embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present application. For those skilled in the art, without creative work, other drawings can be obtained based on these drawings.

[0019] Figure 1 It is a connection schematic diagram of the LCL filter topology in the related art;

[0020] Figure 2 It is a schematic diagram of a filter circuit provided by an embodiment of the present specification;

[0021] Figure 3 It is a specific structure and connection schematic diagram of a filter circuit provided by an embodiment of the present specification;

[0022] Figure 4 It is a schematic diagram of a filter control device provided by an embodiment of the present specification;

[0023] Figure 5 It is a specific connection schematic diagram of the filter control device, generator, and full-power converter provided by an embodiment of the present specification;

[0024] Figure 6 It is a structural schematic diagram of a frequency converter system provided by an embodiment of the present specification;

[0025] Figure 7 It is a specific connection schematic diagram of the filter control device, generator, and doubly-fed converter provided by an embodiment of the present specification. Specific Embodiments

[0026] To make the objectives, technical solutions, and advantages of the embodiments of the present specification clearer, the following will clearly and completely describe the technical solutions of the present application in conjunction with the drawings. Obviously, the described embodiments are some, but not all, of the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of protection of the present application.

[0027] Refer to Figure 1 The connection schematic diagram of the LCL filter topology shown. The grid-side inverter circuit of the converter is usually connected with a filter device. The most common one is that the LC filter circuit is adopted on the bridge arm side for high and low-frequency filtering. The filter device includes a reactor Ls and a filter capacitor C. The reactor Ls, filter capacitor C, and the equivalent leakage inductance Lg of the box transformer form the LCL filter topology. This LCL filter topology can effectively filter out high-frequency harmonics, but has a poor suppression effect on low-frequency harmonics and a narrow filtering band.

[0028] In view of the above problems, some converter manufacturers use a combination of capacitors, inductors, and resistors to suppress harmonics of specific frequencies. However, the existing solutions are diverse, only applicable to specific frequencies, and do not have broadband filtering characteristics.

[0029] Based on this, a filtering circuit, a filtering control device, and an inverter system provided in the embodiments of this specification can be applied to grid-connected filtering.

[0030] For the convenience of understanding this embodiment, a filtering circuit disclosed in the embodiments of this specification will be introduced in detail first, as Figure 2 shown, the filtering circuit includes a first inductor device, a first capacitor device, and multiple filtering branches.

[0031] The first end of the first capacitor device is connected to the first inductor device, and the second end of the first capacitor device is connected to multiple filtering branches; the multiple filtering branches are connected in parallel; Figure 2 Among them, taking three filtering branches as an example, the number of filtering branches in this embodiment is not limited.

[0032] At least some of the filtering branches include resonance parameter adjustment devices, and the resonance parameter adjustment devices in different filtering branches are different; the resonance parameter adjustment device can be a resonance peak adjustment device, for example, a resistor device; the resonance parameter adjustment device can also be a resonance frequency point adjustment device, for example, an inductor device, a capacitor device, etc.

[0033] In actual implementation, a resonance parameter adjustment device can be set in each filtering branch, or only in some of the filtering branches. In the filtering branches without a resonance parameter adjustment device, only wires can be set. In different filtering branches with a resonance parameter adjustment device, the type and parameters of the resonance parameter adjustment device can be different. For example, a resistor device is set in filtering branch 1, and a capacitor device is set in filtering branch 2; or, the same type of device with different parameters can be set in different filtering branches. For example, a capacitor device with a capacitance value of A is set in filtering branch 3, and a capacitor device with a capacitance value of B is set in filtering branch 4.

[0034] In one way, the aforementioned resonance parameter adjustment device includes a resonance peak adjustment device and / or a resonance frequency point adjustment device. Among them, the resonance peak adjustment device and the resonance frequency point adjustment device can be included in multiple filtering branches at the same time, or only one of the resonance peak adjustment device or the resonance frequency point adjustment device can be set.

[0035] It should be noted that different filtering branches can be used to filter out harmonics of different frequency bands. According to the frequency band of the harmonics in the circuit, the corresponding filtering branch of the frequency band is used to filter out the harmonics, so as to achieve broadband harmonic filtering.

[0036] The above filtering branch further includes a switching controller for controlling the connection and disconnection of the filtering branch and the first inductor device.

[0037] The switching controller can be a switching device, a contactor, a circuit breaker, etc. If there is a resonant parameter adjustment device in the filtering branch, usually the resonant parameter adjustment device is connected in series with the corresponding switching controller.

[0038] For a filtering branch, if the switching controller is connected, the resonant parameter adjustment device in the filtering branch is connected to the filtering circuit for harmonic filtering; if the switching controller in the filtering branch is disconnected, the resonant parameter adjustment device in the filtering branch is separated from the filtering circuit and no longer performs harmonic filtering.

[0039] During the operation of the filtering circuit, the switching controller in only one filtering branch can be controlled to be connected, or the switching controllers in multiple filtering branches can be controlled to be connected; by detecting the grid background harmonics, the current harmonic frequency band is determined, and then the corresponding filtering branch is selected for harmonic filtering.

[0040] The above filtering circuit includes a first inductor device, a first capacitor device, and multiple filtering branches; the first end of the first capacitor device is connected to the first inductor device, and the second end of the first capacitor device is connected to the multiple filtering branches; the multiple filtering branches are connected in parallel; at least some of the filtering branches include resonant parameter adjustment devices, and the resonant parameter adjustment devices in different filtering branches are different; the filtering branch further includes a switching controller for controlling the connection and disconnection of the filtering branch and the first inductor device.

[0041] Multiple parallel filtering branches are provided in the filtering circuit, and different resonant parameter adjustment devices are provided in different filtering branches; by controlling the connection and disconnection of the switching controller in the filtering branch, different resonant parameter adjustment devices can be connected, so as to filter out the harmonic waves in the current loop. By connecting different resonant parameter adjustment devices, the harmonic waves in each frequency band can be effectively filtered, with a wide-band filtering function, improving the harmonic filtering effect in each frequency band.

[0042] In a specific implementation manner, if the filtering branch includes a resonant parameter adjustment device, the resonant parameter adjustment device in the filtering branch includes: a resistor device for reducing the resonant peak; or a second inductor device for reducing the resonant frequency point; or a second capacitor device for increasing the resonant frequency point.

[0043] Specifically, when the filtering branch includes a resonant parameter adjustment device, only one resonant parameter adjustment device or only one type of resonant parameter adjustment device is included in one filtering branch. When the resonant parameter adjustment device includes a resistor device, the resistor device can be arranged in one filtering branch or in multiple filtering branches; the resistance value of the resistor device can be determined by the nth and nearby band harmonics detected in the current grid background harmonics. When one filtering branch includes a resistor device, it can include one resistor device or multiple resistor devices, and the multiple resistor devices can be connected in parallel or in series.

[0044] When the resonant parameter adjustment device includes a second inductor device, the second inductor device can be arranged in one filtering branch or in multiple filtering branches; the inductance value of the second inductor device can be determined by the nth and nearby band harmonics detected in the current grid background harmonics. When one filtering branch includes a second inductor device, it can include one second inductor device or multiple second inductor devices, and the multiple second inductor devices can be connected in parallel or in series.

[0045] Similarly, when the resonant parameter adjustment device includes a second capacitor device, the second capacitor device can be arranged in one filtering branch or in multiple filtering branches; the capacitance value of the second capacitor device can be determined by the nth and nearby band harmonics detected in the current grid background harmonics. When one filtering branch includes a second capacitor device, it can include one second capacitor device or multiple second capacitor devices, and the multiple second capacitor devices can be connected in parallel or in series.

[0046] In a specific implementation manner, referring to Figure 3 , the filtering branch includes multiple types of the following: the first branch, the first branch includes a first on-off control device S1 and a resistor device Rc; the first on-off control device and the resistor device are connected in series; the second branch, the second branch includes a second on-off control device S2 and a second inductor device Lc; the second on-off control device and the second inductor device are connected in series; the third branch, the third branch includes a third on-off control device S3 and a second capacitor device C2; the third on-off control device and the second capacitor device are connected in series; the fourth branch, the fourth branch includes a fourth on-off control device S.

[0047] Among them, the first on-off control device, the second on-off control device, the third on-off control device, and the fourth on-off control device can be switch equipment or circuit breakers, etc. The above-mentioned first capacitor device C1 includes an AC capacitor.

[0048] Referring to Figure 4Schematic diagram of the filtering control device shown; the filtering control device includes a filtering circuit and also includes a control device; the control device is used to connect to the power grid and detect the harmonics of the power grid, and the control device is connected to the filtering circuit and controls the on / off of the filtering branches in the filtering circuit.

[0049] The control device may include a DSP (Digital Signal Processing) device, and may also include devices such as a single-chip microcomputer and an ARM (Advanced RISC Machines, RISC microprocessor).

[0050] Since the control device is connected to the power grid, it can collect signals such as the power grid voltage, perform FFT (Fast Fourier Transform) or other frequency analyses on the signals, obtain the harmonics in the power grid; and control the on / off of the filtering branches based on the power grid harmonics.

[0051] Specifically, the above control device is connected to the on / off control devices in each filtering branch of the filtering circuit and is used to control the on / off of the on / off control devices. When the on / off control device in a filtering branch is connected, the resonance parameter adjustment device in that filtering branch is connected to the filtering circuit; the control device determines which resonance parameter adjustment device needs to be connected to the filtering circuit based on the detected harmonics, and then controls the on / off control device corresponding to that resonance parameter adjustment device to be connected.

[0052] See Figure 5 Specific connection schematic diagram of the filtering control device, generator and full-power converter shown. Taking the DSP as an example for the control device, one end of the control device is connected to each filtering branch in the filtering circuit, specifically, it can be connected to the on / off control device in the filtering branch, and the other end of the control device is connected to the power grid; the other end of the control device can be specifically connected between the frame circuit breaker Q1 and the power grid.

[0053] Among them, Ls is the first inductor device, also known as the grid-side filtering inductor; Lg is the equivalent leakage inductance of the box transformer. Each filtering branch includes a switching device and a damping device. The first branch is a damping loop, and its function is to reduce the resonance peak; the second branch is used to lower the resonance frequency point, such as setting an inductor in the second branch; the third branch is used to increase the resonance frequency point, such as setting a capacitor in the second branch; the fourth branch is a conventional filtering switching loop, which only contains a switching device and does not contain a resonance parameter adjustment device. By controlling the switching of the four filtering branches, different filtering topologies can be formed, which can play an inhibitory role for both high-frequency and low-frequency harmonics and avoid resonance faults of the converter.

[0054] See Figure 6Schematic structural diagram of the frequency converter system shown. The frequency converter system includes a frequency converter and the aforementioned filtering control device; the filtering control device is arranged between the frequency converter and the power grid. The frequency converter may specifically include one of a full-power converter or a doubly-fed converter.

[0055] Specifically, the first inductor device in the above filtering control device is connected to the frequency converter, and the controller device is connected to the power grid. The first inductor device in the filtering control device may also be connected to the power grid. The specific connection sequence may be that one end of the frequency converter is connected to one end of the first inductor device, the other end of the first inductor device is connected to one end of the first capacitor device, the other end of the first capacitor device is connected to one end of each filtering branch in the filtering circuit, the other end of each filtering branch is connected to one end of the controller device, and the other end of the controller device is connected to the power grid.

[0056] In a specific implementation manner, the frequency converter includes a doubly-fed converter, and the first capacitor device in the filtering control device is also connected to the generator; the generator is also connected to the doubly-fed converter.

[0057] See Figure 7 Specific connection schematic diagram of the filtering control device, the generator and the doubly-fed converter shown. For the doubly-fed converter and the full-power converter, the filtering control methods in the operating condition and the standby condition are the same. The filtering control method for the doubly-fed converter is described below.

[0058] Step 701, in the operating condition of the doubly-fed converter, control the fourth on-off controller device S to be connected, and disconnect the first on-off controller device S1, the second on-off controller device S2, and the third on-off controller device S3;

[0059] Refer to Figure 7 , when the fourth on-off controller device S is connected and the first on-off controller device S1, the second on-off controller device S2, and the third on-off controller device S3 are disconnected, the filtering circuit is a conventional LCL filtering topology.

[0060] Among them, the first branch is composed of S1 and the resistor device Rc in series; the second branch is composed of S2 and the second inductor device Lc; the third branch is composed of S3 and the second capacitor device C2; the fourth branch is composed of S alone. The first branch, the second branch, and the third branch avoid resonance by switching. The doubly-fed converter is divided into an operating condition and a standby condition, and different conditions have different control methods.

[0061] Step 702, determine the first resonance frequency point of the filtering circuit;

[0062] In one way, the first resonance frequency point can be determined by the following formula:

[0063]

[0064] Wherein, fn is the first resonance frequency point, Lg is the equivalent leakage inductance of the box-type transformer, Ls is the inductance of the first inductor device, also known as the grid-side filter inductance, and C1 is the capacitance of the first capacitor device.

[0065] It should be noted that since the configurations of the doubly-fed converters corresponding to different wind farms are different, therefore, the aforementioned equivalent leakage inductance of the box-type transformer, the first power grid device, and the first capacitor device are usually also different, and thus the resonance frequency points fn of different wind farms are also different.

[0066] Step 703: Collect the grid background harmonics, and based on the grid background harmonics and the first resonance frequency point, determine whether the doubly-fed converter resonates;

[0067] The grid voltage signal can be collected by a control device, and frequency analysis, such as FFT analysis, is performed on the grid voltage signal to obtain the grid background harmonics. Specifically, the harmonic distortion of the harmonics in the specified order and the nearby frequency bands can be detected from the grid background harmonics. The specified order can be represented by n. The specified order n corresponding to different wind farms may be different, and there is a corresponding harmonic distortion threshold preset for different specified orders n. If the harmonic distortion is large and exceeds the corresponding harmonic distortion threshold, and the first resonance frequency point is located in the frequency band of the harmonics in the specified order and the nearby frequency bands, it can be determined that the doubly-fed converter resonates.

[0068] Step 704: If the doubly-fed converter resonates, control the fourth on-off controller device to disconnect, and control the target on-off controller device to connect; wherein, the target on-off controller device includes one or more of the first on-off controller device, the second on-off controller device, and the third on-off controller device.

[0069] For example, if it is necessary to reduce the resonance peak, control the first on-off controller device S1 to close and connect the resistor device to the filter circuit; if it is necessary to lower the resonance frequency point, control the second on-off controller device S2 to close and connect the second inductor device to the filter circuit; if it is necessary to increase the resonance frequency point, control the third on-off controller device S3 to close and connect the second capacitor device to the filter circuit.

[0070] Alternatively, it is also possible to control multiple on-off controller devices among the first on-off controller device, the second on-off controller device, and the third on-off controller device to close, and connect multiple resonance parameter adjustment devices to the filter circuit, so as to more effectively filter out harmonics.

[0071] Furthermore, detect the harmonics in the frequency band of the specified order from the grid background harmonics; if the harmonic distortion of the harmonics in the frequency band of the specified order is greater than the preset harmonic distortion threshold, and the harmonics in the frequency band of the specified order cover the first resonance frequency point, it is determined that the doubly-fed converter resonates.

[0072] To determine that a resonance occurs in the doubly-fed converter, two conditions need to be satisfied simultaneously. Condition 1 is that the harmonic distortion of the harmonic in the specified frequency band is greater than the preset harmonic distortion threshold. Specifically, the THD (Total Harmonic Distortion) of the harmonic in the detected specified order n and the adjacent frequency band in the grid background harmonic is greater than the preset harmonic distortion threshold. Condition 2 is that the harmonic in the specified frequency band covers the first resonance frequency point, that is, the resonance frequency point of the aforementioned LCL filter topology. When Conditions 1 and 2 are satisfied simultaneously, the doubly-fed converter may resonate, resulting in faults such as over-limit of the filter capacitor and filter current in the doubly-fed converter.

[0073] On the premise that the fourth on-off control device S is connected and the first on-off control device S1, the second on-off control device S2, and the third on-off control device S3 are disconnected, when it is determined that the doubly-fed converter resonates, first control the fourth on-off control device S to disconnect and control the second on-off control device S2 to connect.

[0074] In this case, the filter circuit is an L-LC-L type filter topology. Since the filtering point changes, the resonance frequency point is updated from the aforementioned first resonance frequency point to the second resonance frequency point, and the second resonance frequency point can be calculated by the following formula:

[0075]

[0076] f n1 is the second resonance frequency point, Lg is the equivalent leakage inductance of the transformer substation, Ls is the inductance of the first inductor device, also known as the grid-side filter inductance, Lc is the inductance of the second inductor device, and C1 is the capacitance of the first capacitor device. By changing the resonance frequency point, resonance can be avoided.

[0077] Obtain the second resonance frequency point of the filter circuit after the second on-off control device is connected; based on the grid background harmonic and the second resonance frequency point, determine whether the doubly-fed converter resonates; if the doubly-fed converter resonates, control the first on-off control device to connect.

[0078] In actual implementation, detect the harmonic in the specified frequency band from the grid background harmonic; if the harmonic distortion of the harmonic in the specified frequency band is greater than the preset harmonic distortion threshold and the harmonic in the specified frequency band covers the second resonance frequency point, determine that the doubly-fed converter resonates, and the resonance of the doubly-fed converter may result in a relatively large THD.

[0079] After S is disconnected and S2 is closed, if the doubly-fed converter still resonates, the first on-off control device S1 also needs to be closed.

[0080] The resonance is suppressed by using a parallel device of a resistive device and a second inductor device. Both the resonance frequency and the resonance gain are changed. In particular, the resonance gain is significantly attenuated. Also, the second inductor device and the resistive device share the current. The low-frequency harmonic components mainly flow through the second inductor device, and the high-frequency harmonic components mainly flow through the resistive device, which can also reduce the thermal loss of the resistive device.

[0081] Furthermore, obtain the third resonance frequency point of the filter circuit when both the first on-off controller device and the second on-off controller device are connected; based on the grid background harmonics and the third resonance frequency point, determine whether the doubly-fed converter resonates; if the doubly-fed converter resonates, control the first on-off controller device and the second on-off controller device to disconnect, and control the third on-off controller device to connect.

[0082] In actual implementation, detect the harmonics in the specified frequency band from the grid background harmonics; if the harmonic distortion of the harmonics in the specified frequency band is greater than the preset harmonic distortion threshold and the harmonics in the specified frequency band cover the third resonance frequency point, determine that the doubly-fed converter resonates. The resonance of the doubly-fed converter may lead to a large THD.

[0083] When the first on-off controller device, the second on-off controller device, and the fourth on-off controller device are all disconnected, only control the third on-off controller device to connect, connect the second capacitor device to the filter circuit, absorb the harmonics through the second capacitor device, and the resonance frequency point changes, thereby avoiding resonance.

[0084] Furthermore, obtain the fourth resonance frequency point of the filter circuit after the third on-off controller device is connected; based on the grid background harmonics and the fourth resonance frequency point, determine whether the doubly-fed converter resonates; if the doubly-fed converter resonates, control the first on-off controller device to connect.

[0085] In actual implementation, detect the harmonics in the specified frequency band from the grid background harmonics; if the harmonic distortion of the harmonics in the specified frequency band is greater than the preset harmonic distortion threshold and the harmonics in the specified frequency band cover the fourth resonance frequency point, determine that the doubly-fed converter resonates. The resonance of the doubly-fed converter may lead to a large THD.

[0086] After the third on-off controller device S3 is closed, if the doubly-fed converter still resonates, the first on-off controller device S1 also needs to be closed. When S1 and S3 are closed simultaneously, a parallel method of a resistive device and a second capacitor device is used to suppress resonance.

[0087] Furthermore, after suppressing the resonance by connecting the target on-off controller device, when it is determined that the harmonic distortion of the harmonics in the specified frequency band in the grid background harmonics is less than or equal to the preset harmonic distortion threshold, control the target on-off controller device to disconnect and control the fourth on-off controller device to connect.

[0088] When the harmonic distortion of the harmonic in the specified frequency band of the grid background harmonics is less than or equal to the preset harmonic distortion threshold, it can be understood that the resonance risk is relatively low. The previously connected S1, S2, or S3 can be disconnected, and only S is connected to continue operating with the LCL filter topology. During the operation, it is possible to monitor whether the doubly-fed converter resonates based on the control device. If resonance occurs, the previously mentioned target on-off control device is continuously controlled to be connected to suppress the resonance.

[0089] In other methods, in the standby condition of the doubly-fed converter, the grid-side circuit breaker, that is, the aforementioned Figure 7 Q1 in usually does not trip. At this time, there are two control methods for the filter circuit.

[0090] Method 1: The stator contactor K1 is disconnected, or the first on-off control device S1, the second on-off control device S2, the third on-off control device S3, and the fourth on-off control device S4 are all disconnected. The stator-side filter topology of the generator is disconnected from the grid, and the grid side operates with a single inductor, and there is no resonance risk.

[0091] Method 2: The filter circuit is still connected to the grid side, and the grid side is in the modulation state or the non-modulation state. The control device still detects the grid voltage in real time, which is the same as the filter control method under the operating condition.

[0092] In this embodiment, the filter circuit is installed on the rotor side of the generator. When it is detected that the harmonics at a certain frequency point of the grid are large and exceed the set threshold, different filter topologies are formed by switching and selecting multiple filter branches, which can not only change the resonance frequency but also greatly reduce the resonance peak value. On the one hand, the filter branches used can be switched and selected according to the existing harmonics in the grid to achieve high-frequency and low-frequency filtering. On the other hand, it can ensure that resonance does not occur in the standby state.

[0093] A computer program product of a filter circuit, a filter control device, and a frequency converter system provided by an embodiment of this specification includes a computer-readable storage medium storing program codes. The instructions included in the program codes can be used to execute the methods described in the foregoing method embodiments. For specific implementation, reference can be made to the method embodiments, which will not be elaborated here.

[0094] Those skilled in the art can clearly understand that for the convenience and brevity of description, the specific working processes of the systems and devices described above can refer to the corresponding processes in the foregoing method embodiments, which will not be elaborated here.

[0095] In addition, in the description of the embodiments of this specification, unless otherwise clearly specified and limited, the terms "installation", "connection", and "coupling" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, and it can be the communication inside two components. For those skilled in the art, the specific meanings of the above terms in this application can be understood according to specific circumstances.

[0096] If the above-mentioned functions are implemented in the form of software function units and sold or used as independent products, they can be stored in a computer-readable storage medium. Based on such an understanding, the technical solution of this application, in essence, or the part that contributes to the prior art, or a part of this technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions for causing a computer device (which can be a personal computer, a server, or a network device, etc.) to execute all or part of the steps of the methods described in the various embodiments of this application. The aforementioned storage medium includes: various media such as USB flash drives, mobile hard disks, read-only memories (ROM, Read-Only Memory), random access memories (RAM, Random Access Memory), magnetic disks, or optical discs that can store program codes.

[0097] In the description of this application, it should be noted that the orientation or positional relationship indicated by the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc. is based on the orientation or positional relationship shown in the drawings. It is only for the convenience of describing this application and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. Therefore, it should not be construed as a limitation to this application. In addition, the terms "first", "second", and "third" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance.

[0098] Finally, it should be noted that the above embodiments are only specific implementation manners of this application, used to illustrate the technical solution of this application, rather than limiting it. The protection scope of this application is not limited thereto. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that: Any person skilled in the art within the technical scope disclosed in this application can still modify the technical solutions recorded in the foregoing embodiments, or can easily think of changes, or perform equivalent replacements on some of the technical features; and these modifications, changes, or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of this specification, and should all be covered within the protection scope of this application. Therefore, the protection scope of this application should be subject to the protection scope of the claims.

Claims

1. A filtering circuit, characterized in that, The filtering circuit includes a first inductor device, a first capacitor device, and a plurality of filtering branches; A first end of the first capacitor device is connected to the first inductor device, and a second end of the first capacitor device is connected to the plurality of filtering branches; the plurality of filtering branches are connected in parallel; At least some of the filtering branches include resonance parameter adjustment devices, and the resonance parameter adjustment devices in different filtering branches are different; The filtering branch further includes a turn-on / off control device for controlling the turn-on and turn-off of the filtering branch and the first inductor device.

2. The filter circuit according to claim 1, wherein The resonance parameter adjustment device includes a resonance peak adjustment device and / or a resonance frequency point adjustment device.

3. The filter circuit according to claim 1, characterized in that, If the filtering branch includes a resonance parameter adjustment device, the resonance parameter adjustment device in the filtering branch includes: A resistor device for reducing the resonance peak; Or, a second inductor device for reducing the resonance frequency point; Or, a second capacitor device for increasing the resonance frequency point.

4. The filter circuit according to claim 1, wherein The filtering branch includes multiple types of the following: A first branch, the first branch includes a first turn-on / off control device and a resistor device; the first turn-on / off control device and the resistor device are connected in series; A second branch, the second branch includes a second turn-on / off control device and a second inductor device; the second turn-on / off control device and the second inductor device are connected in series; A third branch, the third branch includes a third turn-on / off control device and a second capacitor device; the third turn-on / off control device and the second capacitor device are connected in series; A fourth branch, the fourth branch includes a fourth turn-on / off control device.

5. The filter circuit according to claim 1, wherein The first capacitor device includes an AC capacitor.

6. A filtering control device, characterized in that, The filtering control device includes the filtering circuit according to any one of claims 1-5, and further includes a controller device; The controller device is used to connect to the power grid and detect the harmonics of the power grid, and the controller device is connected to the filtering circuit and controls the turn-on and turn-off of the filtering branches in the filtering circuit.

7. The filtering control device according to claim 6, wherein The controller device is connected to the turn-on / off control devices in each filtering branch of the filtering circuit for controlling the turn-on and turn-off of the turn-on / off control devices.

8. A frequency converter system, characterized in that, The frequency converter system includes a frequency converter and the filtering control device according to claim 6 or 7; The filtering control device is arranged between the frequency converter and the power grid.

9. The frequency converter system according to claim 8, characterized in that The first inductor device in the filtering control device is connected to the frequency converter, and the first capacitor device and the controller device in the filtering control device are respectively connected to the power grid.

10. The frequency converter system according to claim 8, characterized in that, The frequency converter includes a full-power converter or a doubly-fed converter.

11. The frequency converter system according to claim 8, characterized in that, The frequency converter includes a doubly-fed converter, and the first capacitor device in the filtering control device is further connected to a generator; the generator is also connected to the doubly-fed converter.

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