A low pass filter device

Abstract

The present invention discloses a low-pass filter device, including an input terminal, a capacitor, an output terminal, a metal shell, a first reactor, a second reactor and a third reactor; the output terminal is located on one side of the metal shell, and the input terminal is connected to a current transformer through a secondary circuit of a wire; the capacitor and the first reactor, the second reactor and the third reactor form a low-pass filter circuit in a series-parallel manner; the output terminal is located on the other side of the metal shell, and is connected to a receiving module through a wire. The device eliminates the influence of high-order harmonic current on sampling through the filtering performance of the low-pass filter, filters out high-order harmonic current through hardware, and ensures that the sampling is protected from the influence of high-order harmonic current; it has no influence on the magnitude of the fundamental current, and ensures the accuracy of sampling.

Description

Technical Field

[0001] The present invention relates to the technical field of high-voltage flexible alternating current power transmission systems, and in particular to a low-pass filter device used in a sampling loop of a series-type flexible alternating current power transmission system. Background Art

[0002] With the development of economy, the power load is growing continuously, which puts forward higher requirements on the transmission capacity of power grid. In particular, the demand for power system flow control, voltage control and short-circuit current control is increasing, so the application of Flexible AC Transmission Systems (FACTS) devices in power systems is increasing.

[0003] FACTS can be divided into series devices, parallel devices and series-parallel devices according to the way they are connected to the power grid.

[0004] The currently used series flexible AC transmission devices include: series compensation devices and series resonant fault current limiting devices, the schematic diagram of which is shown in the figure. Figure 1 As shown. In the figure, 31 is the busbar on one side of the transmission line; 36 is the busbar on the other side of the transmission line, 32 is the line switch on one side of the transmission line; 35 is the line switch on the other side of the transmission line, 33 is the transmission line, and 34 is the installed series device. Since the device is connected in series in the line, its insulation requirements to the ground are consistent with the insulation requirements of the line to the ground. In order to reduce the project cost, the series device is installed on an insulating platform of a steel structure, and the insulating platform is fixed by support insulators and inclined insulators. Therefore, there is no electrical connection between the series device and the ground. The insulation requirements are met by the support insulators and inclined insulators, and the device only needs to consider the insulation requirements for the insulating platform, which greatly reduces the insulation level of the series device itself.

[0005] Considering that the series flexible AC transmission device is connected in series in the line, in order to ensure the normal operation of the main equipment, overvoltage protection equipment needs to be configured. Corresponding secondary protection is also required for these primary main equipment. Then, the corresponding electrical quantities on the insulating platform need to be collected and transmitted to the protection device on the ground as the basis for the protection device to make logical judgments.

[0006] For series compensation devices, their secondary protection includes capacitor unbalance protection. During the operation of a domestic series compensation device after adding DC and SVG to the surrounding system, the capacitor unbalance protection action occurred, causing the series compensation device to exit. In the subsequent fault analysis, no abnormal phenomenon was found in the capacitor unit of the series compensation device, but when analyzing the recorded waveform, it was found that the power frequency component of the capacitor unbalanced current was very small, while the high-order harmonics contained a large amount. Since the algorithm used by the secondary control protection device equipped with the series compensation device is the root mean square algorithm, it is sensitive to high-order harmonics, which caused the protection to malfunction. Considering that the manufacturer no longer has after-sales technical support in China, it is impossible to eliminate the impact of high-order harmonic currents on protection by upgrading the software, and the filtering devices used in the power system in the past generally filter the voltage signal. Summary of the invention

[0007] The object of the present invention is to overcome the above-mentioned deficiencies in the prior art and provide a low-pass filter device.

[0008] The present invention is realized through the following technical scheme: a low-pass filter device, comprising an input terminal, a capacitor, an output terminal, a metal shell, a first reactor, a second reactor and a third reactor; the input terminal is located on one side of the metal shell, and the input terminal is connected to a current transformer for collecting the current of the primary circuit through a secondary circuit of a wire; the output terminal is located on the other side of the metal shell, and is connected to a receiving module through a wire; one end of the first reactor is connected to an input terminal on one side, and the other end is connected to one end of the second reactor, and the other end of the second reactor is connected to the output terminal on this side; the capacitor is respectively connected to the input terminal on the other side and the output terminal on the other side through a wire, the capacitor is connected to one end of the third reactor, and the other end of the third reactor is connected to the connecting line between the first reactor and the second reactor; the capacitor and the first reactor, the second reactor, and the third reactor constitute a low-pass filter circuit.

[0009] The first reactor forms a first branch, the second reactor forms a second branch, the third reactor and the capacitor are connected in series to form a third branch, and the reactor and the capacitor form a series-parallel connection to form a low-pass filter circuit.

[0010] The low-pass filter loop adopts an elliptic function low-pass network, and its cut-off frequency is 400 Hz.

[0011] The low-pass filter circuit is connected in series with the sampling circuit. The low-pass filter circuit is connected in series with the sampling circuit to filter out high-order harmonic currents.

[0012] The current transformer is a through-type current transformer.

[0013] Compared with the prior art, the advantages of the present invention are: the device eliminates the influence of high-order harmonic currents on sampling through the filtering performance of the low-pass filter, and is suitable for the current sampling circuit in the power system; the high-order harmonic currents are filtered out by hardware to ensure that the sampling is protected from the influence of high-order harmonic currents; the magnitude of the fundamental current is not affected, thereby ensuring the accuracy of the sampling. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a structural schematic diagram of the existing series flexible AC transmission device connected to the power system;

[0015] Figure 2 It is a schematic diagram of the structure of the low-pass filter of the present invention;

[0016] Figure 3 It is a schematic diagram of the structure of the low-pass filter connected to the sampling loop of the present invention.

[0017] The meanings of the reference numerals in the figure are as follows: 1. low-pass filter; 11. first inductor; 12. second inductor; 13. third inductor; 14. capacitor; 15. input terminal; 16. output terminal; 17. metal casing; 21. first capacitor bank branch; 22. second capacitor bank branch; 23. first through-type current transformer; 24. second through-type current transformer; 25. sampling device; 31. busbar on one side of the transmission line; 32. line switch on one side of the transmission line; 33. transmission line; 34. installed series device; 35. line switch on the other side of the transmission line; 36. busbar on the other side of the transmission line. DETAILED DESCRIPTION

[0018] The content of the present invention is further described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0019] Example

[0020] Reference Figure 2, which is a low-pass filter device, including an input terminal 15, a capacitor 14, an output terminal 16, a metal shell 17, a first reactor 11, a second reactor 12 and a third reactor 13; the input terminal 15 is located on one side of the metal shell 17, and the input terminal 15 is connected to a current transformer for collecting the current of the primary circuit through a wire secondary circuit; the output terminal 16 is located on the other side of the metal shell 17, and is connected to a receiving module through a wire; one end of the first reactor 11 is connected to the input terminal 15 on one side, and the other end of the first reactor 11 is connected to the input terminal 15 on one side, and the second end of the first reactor 11 is connected to the input terminal 15 on the other side. The first end of the second reactor 12 is connected to one end of the second reactor 12, and the other end of the second reactor 12 is connected to the output terminal 16 of this side; the capacitor 14 is respectively connected to the input terminal 15 on the other side and the output terminal 16 on the other side through wires, the capacitor 14 is connected in series with one end of the third reactor 13, and the other end of the third reactor 13 is connected to the connecting line between the first reactor 11 and the second reactor 12; the capacitor 14 and the first reactor 11, the second reactor 12, and the third reactor 13 form a low-pass filter circuit in a series-parallel manner.

[0021] The first reactor 11 forms a first branch, the second reactor 12 forms a second branch, the third reactor 13 and the capacitor 14 are connected in series to form a third branch, and the reactor and the capacitor 14 form a series-parallel connection to form a low-pass filter circuit.

[0022] The low-pass filter circuit adopts an elliptic function low-pass network with a cut-off frequency of 400Hz.

[0023] The low-pass filter circuit is connected in series with the sampling circuit. The low-pass filter circuit is connected in series to filter out high-order harmonic currents.

[0024] The current transformer is a through-type current transformer.

[0025] In this embodiment, the receiving module is an existing general technology and will not be analyzed in detail.

[0026] See also Figure 3 When in use, the low-pass filter 1 is connected in series to the secondary terminals of the first through-type current transformer 23 and the second through-type current transformer 24, the first capacitor group branch 21 is connected in series with the first through-type current transformer 23 to form a first branch, the second capacitor group branch 22 is connected in series with the second through-type current transformer 24 to form a second branch, and the first branch is connected in parallel with the second branch; the other end of the low-pass filter 1 is connected to the sampling loop of the sampling device 25. When high-order harmonic components appear in the unbalanced current of the capacitor 14, the high-order harmonic current can be filtered out by the low-pass filter 1. The cut-off frequency selected in this application example is 400Hz, which can meet the filtering requirements of the high-order harmonic current in this example. If there are other requirements for the cut-off frequency, it can be achieved by modifying the reactor and capacitor 14 of the filter circuit.

[0027] The device provided by this device can effectively filter out the high-order harmonic current components generated in the capacitor bank, ensure the readiness of protection sampling, and keep the series compensation device in normal operation.

[0028] The above detailed description is a specific description of a feasible embodiment of the present invention. The embodiment is not intended to limit the patent scope of the present invention. Any equivalent implementation or modification that does not deviate from the present invention should be included in the patent scope of this case.

Claims

1. A low-pass filter device, characterized in that: The invention comprises an input terminal, a capacitor, an output terminal, a metal shell, a first reactor, a second reactor and a third reactor; the input terminal is located on one side of the metal shell, and the input terminal is connected to a current transformer for collecting the current of the primary circuit through a secondary circuit of a wire; the output terminal is located on the other side of the metal shell, and is connected to a receiving module through a wire; one end of the first reactor is connected to an input terminal on one side, and the other end is connected to one end of the second reactor, and the other end of the second reactor is connected to the output terminal on this side; the capacitor is respectively connected to the input terminal on the other side and the output terminal on the other side through a wire, and the capacitor is connected to one end of the third reactor, and the third The other end of the reactor is connected to the connecting line between the first reactor and the second reactor; the capacitor and the first reactor, the second reactor, and the third reactor form a low-pass filter circuit; the low-pass filter circuit is connected in series to the sampling circuit; the current transformer adopts a through-type current transformer; the low-pass filter is connected in series to the secondary terminals of the first through-type current transformer and the second through-type current transformer, the first capacitor group branch is connected in series with the first through-type current transformer to form a first branch, the second capacitor group branch is connected in series with the second through-type current transformer to form a second branch, and the first branch is connected in parallel with the second branch; the other end of the low-pass filter is connected to the sampling circuit of the sampling device.

2. The low-pass filter device according to claim 1, characterized in that: The low-pass filter loop adopts an elliptic function low-pass network, and its cut-off frequency is 400 Hz.

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