An adaptive composite filtering method and device for mixed interference of weak current loop

Abstract

This invention discloses an adaptive composite filtering method and device for mixed interference in low-voltage circuits, relating to the field of electromagnetic interference suppression. Addressing the problem that existing fixed-parameter capacitor filtering schemes cannot adapt to dynamic changes in interference frequencies in the field, and that common-mode circuits easily introduce power frequency circulating currents and surge risks, this invention constructs a variable capacitor network between signal lines and between lines and ground. A dynamic spectrum detection unit extracts noise characteristic parameters in real time and adaptively adjusts the equivalent capacitance according to a dynamic mapping strategy. Simultaneously, a bidirectional transient voltage suppression unit is connected in series between the common-mode capacitor and the protective ground. Utilizing its nonlinear impedance characteristics, it blocks low-frequency power frequency circulating currents, conducts high-frequency common-mode discharge paths, and suppresses transient surges. This invention achieves adaptive suppression of complex mixed interference while ensuring signal integrity, significantly improving the transmission stability and safety of low-voltage circuits in harsh electromagnetic environments.

Description

Technical Field

[0001] This invention relates to the field of electromagnetic compatibility and signal processing technology in power systems, specifically to an adaptive composite filtering method and apparatus for handling mixed high-frequency interference in transmission circuits of multiple types of weak electrical signals under complex electromagnetic environments. Background Technology

[0002] In field environments such as power generation and industrial process control, low-voltage signal loops such as 4-20mA analog current loops, 0-10V voltage signals, RS485 digital communication buses, thermocouples (TC), and resistance temperature detectors (RTDs) are widely used for parameter acquisition and equipment control. Due to the dense concentration of high-power electrical equipment and complex wiring in the field, low-voltage loops are highly susceptible to electromagnetic interference from equipment such as frequency converters, switching power supplies, and welding machines, manifesting as differential-mode high-frequency interference and common-mode high-frequency interference superimposed on the useful signal.

[0003] To suppress the aforementioned interference, a common practice in the prior art is to introduce a passive filter network consisting of fixed-capacity capacitors into the low-voltage circuit. For example, a capacitor is connected in parallel between the positive and negative terminals of the signal line to filter out differential-mode interference, while a capacitor is connected in parallel between the signal line and the protective ground to filter out common-mode interference.

[0004] However, through long-term on-site operation and maintenance and signal testing, the applicant discovered that the above-mentioned passive filtering scheme based on fixed parameters has the following two prominent technical problems in practical applications:

[0005] First, there is an irreconcilable contradiction between filtering effectiveness and signal integrity. The frequency of electromagnetic interference in the field is not fixed but dynamically drifts with the start-up and shutdown of high-power equipment and load changes. If a large-capacity capacitor (such as 0.1μF) is selected to suppress potential low-frequency interference, although it can effectively bypass low-frequency noise, during periods of light or no interference, the large capacitor will severely bypass the useful high-frequency components of high-speed signals such as RS485, resulting in slower signal edges and increased inter-symbol interference, which actually reduces communication reliability. Conversely, if a small-capacity capacitor (such as 1000pF) is selected to protect high-speed signals, its ability to suppress mid- and low-frequency interference is significantly insufficient. This contradiction between "static filtering parameters" and "dynamic interference spectrum" is the fundamental reason why existing fixed-parameter filtering schemes are ineffective in field applications and even introduce new problems.

[0006] Secondly, the ground loop interference and surge risks introduced by common-mode filter capacitors are generally overlooked. When a common-mode filter capacitor is connected between the positive and negative terminals of the signal line and the protective ground, there is usually a certain low-frequency potential difference (mainly 50Hz power frequency and its harmonics) between the power field protective ground (PE) and the signal reference ground, and there may even be transient surge overvoltages. The fixed common-mode capacitor will provide a low-impedance path for these low-frequency or transient currents. This will lead to two serious consequences: First, the power frequency current flows into the signal loop through the common-mode capacitor and is converted into differential-mode interference voltage on the loop impedance, i.e., the so-called "ground loop interference", which seriously affects the zero-point accuracy and stability of analog signals; second, in the event of lightning strikes or operational overvoltages, surge currents will be directly injected into the low-voltage interface through the common-mode capacitor, which can easily cause permanent damage to the interface chip.

[0007] Therefore, there is an urgent need for a filtering scheme that can both adaptively match the dynamically changing interference spectrum on site and effectively block power frequency circulating current and suppress surge impact. Summary of the Invention

[0008] The present invention aims to overcome the above-mentioned shortcomings of the prior art and provide an adaptive composite filtering method and device for mixed interference in weak current circuits, so as to solve the technical problems that the existing fixed parameter filtering schemes cannot simultaneously take into account dynamic interference suppression and signal integrity, and that common mode current is prone to introducing power frequency circulating current and surge risks.

[0009] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0010] An adaptive composite filtering method for mixed interference in low-voltage circuits, applied to low-voltage signal transmission circuits containing a signal line positive terminal, a signal line negative terminal, and a protective ground, includes the following steps:

[0011] A first variable capacitor network is set between the positive and negative terminals of the signal line to bypass differential mode interference signals.

[0012] A second switched capacitor network is set between the positive terminal of the signal line and the protective ground, and a third switched capacitor network is set between the negative terminal of the signal line and the protective ground to bypass common-mode interference signals.

[0013] Real-time acquisition of noise characteristic parameters in low-voltage signal transmission loops;

[0014] Based on the noise characteristic parameters, the equivalent capacitance of the first variable capacitor network, the second switched capacitor network, and the third switched capacitor network are adjusted synchronously according to a preset dynamic mapping strategy.

[0015] Specifically, a bidirectional transient voltage suppression unit is connected in series between the second switched capacitor network and the protective ground, and between the third switched capacitor network and the protective ground. The bidirectional transient voltage suppression unit is configured to: present a high impedance state for low-frequency voltages below its conduction threshold to block power frequency circulating current, and present a low impedance state for high-frequency common-mode interference or surge voltages above its conduction threshold to conduct bypass paths and clamp protection.

[0016] Furthermore, the dynamic mapping strategy includes:

[0017] When the main peak frequency of the noise spectrum is detected to be higher than the first preset threshold, the equivalent capacitance of the first variable capacitor network, the second switched capacitor network and the third switched capacitor network are synchronously adjusted to the first capacitance range.

[0018] When the main peak frequency of the noise spectrum is detected to be lower than the second preset threshold, the equivalent capacitance of the first variable capacitor network, the second switched capacitor network and the third switched capacitor network are synchronously adjusted to the second capacitance range.

[0019] The value of the first capacity range is smaller than the value of the second capacity range.

[0020] Furthermore, the first capacity range is 1000pF to 2200pF, and the second capacity range is 0.01μF to 0.1μF.

[0021] Furthermore, the step of real-time acquisition of noise characteristic parameters includes: extracting the common-mode voltage fluctuation spectrum or differential-mode voltage fluctuation spectrum superimposed on the useful signal in the weak electrical signal transmission circuit through a high-impedance sampling unit.

[0022] Furthermore, the bidirectional transient voltage suppression unit is a bidirectional transient voltage suppression diode.

[0023] Furthermore, the weak current signal transmission circuit is a circuit that transmits any one of the following signals: 4-20mA current signal, 0-10V voltage signal, RS485 communication signal, thermocouple signal, or resistance temperature detector signal.

[0024] An adaptive composite filtering device for mixed interference in low-voltage circuits, used for connection in a low-voltage signal transmission circuit including a positive terminal of a signal line, a negative terminal of a signal line, and a protective ground, comprising:

[0025] The first variable capacitor network is connected in parallel between the positive and negative terminals of the signal line.

[0026] The second switched capacitor network has one end connected to the positive terminal of the signal line;

[0027] The third switched capacitor network has one end connected to the negative terminal of the signal line;

[0028] The dynamic spectrum detection and control unit has its input terminal coupled to the positive and / or negative terminals of the signal line. It is used to collect noise characteristic parameters in real time and output control signals according to a preset dynamic mapping strategy to adjust the equivalent capacitance of the first variable capacitor network, the second switched capacitor network and the third switched capacitor network.

[0029] The first bidirectional transient voltage suppression unit is connected in series between the other end of the second switched capacitor network and the protective ground;

[0030] The second bidirectional transient voltage suppression unit is connected in series between the other end of the third switched capacitor network and the protective ground.

[0031] Furthermore, both the first bidirectional transient voltage suppression unit and the second bidirectional transient voltage suppression unit are bidirectional transient voltage suppression diodes.

[0032] Furthermore, the first variable capacitor network includes a voltage-controlled capacitor array composed of varactor diodes, or a switched capacitor array composed of multiple fixed capacitors and controllable switches.

[0033] Furthermore, the dynamic spectrum detection and control unit includes an impedance transformation circuit, a bandpass filter circuit, an analog-to-digital conversion circuit, and a microcontroller connected in sequence.

[0034] Compared with the prior art, the beneficial effects of the present invention are:

[0035] 1. This invention achieves real-time tracking and adaptive matching of filtering parameters to the field interference spectrum through the closed-loop linkage of dynamic spectrum detection and variable capacitor network. When the interference is severe, the filtering depth is automatically increased, and when the interference is calm, the capacitance is automatically reduced to protect the high-speed signal waveform. This intelligent adjustment capability is not available in any fixed parameter filtering network, and fundamentally solves the contradiction of "paying attention to one thing but losing another".

[0036] 2. This invention creatively uses a bidirectional transient voltage suppression unit as an "intelligent ground switch" for the common-mode capacitor. Utilizing its nonlinear impedance characteristics, it naturally blocks the power frequency circulating current during normal operation, only opening the discharge path when high-frequency interference or surges occur. Field tests show that this design can reduce the amplitude of 50Hz series-mode interference introduced by the protective ground potential difference by more than 90%, greatly improving the zero-point stability of analog signal acquisition.

[0037] 3. The bidirectional transient voltage suppression unit in series in this invention is itself a standard surge protection device. This solution achieves filtering functionality without requiring additional protection components, thus naturally constructing an interface surge barrier with extremely fast response and strong current-carrying capacity, avoiding the risk of accidentally reducing the port's immunity level due to the connection of ordinary capacitors;

[0038] 4. The entire device of the present invention can be composed of a small number of electronic components, is compact in size, and can be directly connected to the terminal block of the existing circuit without the need for large-scale modification of the original cable wiring, making it very suitable for upgrading and deploying existing power production sites. Attached Figure Description

[0039] Figure 1 The circuit principle block diagram of the adaptive composite filter device provided in the embodiment of the present invention applied to the weak current signal transmission loop.

[0040] In the diagram: 100 - Low-voltage signal transmission circuit;

[0041] 101 - Signal line positive terminal (Signal+);

[0042] 102 - Signal line negative terminal (Signal-);

[0043] 103 - Protected Area (PE);

[0044] 200-Adaptive Composite Filter Device;

[0045] 210 - Dynamic Spectrum Detection and Control Unit;

[0046] 220 - First variable capacitor network;

[0047] 230 - Second switched capacitor network;

[0048] 240 - Third switched capacitor network;

[0049] 250 - First bidirectional transient voltage suppression unit;

[0050] 260 - Second bidirectional transient voltage suppression unit. Detailed Implementation

[0051] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0052] Example 1: RS485 communication bus application scenario

[0053] An RS485 bus (115200bps baud rate) in a substation is used to collect data from multiple smart meters. Welding machines are frequently used for maintenance on-site, causing a sharp increase in the bus error rate when the welding machines are operating. Furthermore, the bus often enters a locked state, requiring manual power-off reset to recover.

[0054] Problem Diagnosis: On-site oscilloscope measurements showed that when the welding machine was working, high-intensity common-mode interference pulses with a frequency of over 10MHz were coupled onto the A and B lines of the RS485. At the same time, there was a power frequency (50Hz) potential difference of about 3V between the protective ground (PE) and the equipment signal ground.

[0055] The solution adopted by the present invention:

[0056] Device Connection: Connect the device 200 of this invention to the hub input terminal of the RS485 bus. Figure 1 Connect the circuit as shown.

[0057] Component selection:

[0058] The first, second, and third capacitor networks are selected from switched capacitor arrays controlled by analog switches, each containing three levels: 1000pF, 2200pF, and 0.01μF.

[0059] The first and second bidirectional transient voltage suppression units 250 and 260 are bidirectional TVS diodes (model SMBJ6.0CA) with a breakdown voltage of 6.8V.

[0060] The dynamic spectrum detection and control unit 210 employs a low-power microcontroller with built-in ADC and FFT computing capabilities.

[0061] Work process:

[0062] When the welding machine is not working, the circuit noise level is low. The control unit 210 detects that the main interference frequency is higher than 1MHz but the energy is weak. It configures the capacitor network to the minimum level of 1000pF to ensure that the waveform edge of the RS485 signal is not affected and the communication is stable.

[0063] At the moment the welding machine starts its arc, the control unit 210 detects high-intensity common-mode noise above 10MHz and switches the capacitor network to the 2200pF setting to effectively bypass the high-frequency noise.

[0064] Throughout the operation, due to the 3V power frequency potential difference between the protective ground and the ground, which is lower than the breakdown voltage (6.8V) of TVS diodes 250 and 260, the TVS diodes exhibit a high resistance state, successfully blocking the path of power frequency current flowing into lines A and B through capacitor networks 230 and 240, thus avoiding bus lock-up faults caused by ground loops.

[0065] Implementation Results: After installing this device, the RS485 communication error rate during welding machine operation decreased from 10... -4 The magnitude dropped to 10 -7 The system has been running continuously for three months without any further bus lock-up issues.

[0066] Example 2: Application scenario of 4-20mA analog input loop

[0067] The temperature of the turbine bearings in a thermal power plant is measured using a PT100 resistance temperature detector (RTD), and the signal is converted into a 4-20mA current signal and sent to the DCS system via a transmitter. After the generator excitation system is put into operation, the temperature value displayed on the DCS shows a periodic fluctuation of about 0.5%, triggering a false alarm from the protection system.

[0068] Problem Diagnosis: Spectrum analysis revealed a common-mode interference voltage with a frequency of approximately 80kHz and varying amplitude superimposed on the 4-20mA circuit. This interference originates from the switching frequency of the excitation system and is coupled to the signal circuit through the distributed capacitance between cables.

[0069] The solution adopted by the present invention:

[0070] Device connection: Connect the device 200 of the present invention in parallel to the terminal of the DCS analog input card.

[0071] Component selection: The capacitor network is configured with two levels: 0.01μF and 0.1μF.

[0072] The TVS diode should be selected with a breakdown voltage slightly higher than the maximum common-mode voltage of the circuit.

[0073] Work process:

[0074] After the excitation system was put into operation, the control unit 210 detected a significant increase in 80kHz common-mode noise energy.

[0075] According to the dynamic mapping strategy, the control unit 210 switches the second and third switched capacitor networks 230 and 240 to the 0.01μF level.

[0076] The 80kHz high-frequency common-mode current is successfully bypassed to the protective ground through the TVS transistor (which now exhibits low impedance due to the high frequency and high voltage) and the 0.01μF capacitor.

[0077] At the same time, the TVS tube blocks power frequency and low frequency noise on the protective ground.

[0078] Implementation results: After the device was installed, the 4-20mA signal waveform displayed by the DCS became smooth, the 80kHz ripple was effectively filtered out, the temperature display fluctuations disappeared, and the problem of false alarms was completely solved.

[0079] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art can make various improvements and modifications without departing from the spirit and principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. An adaptive composite filtering method for mixed interference in weak current circuits, applied to a weak current signal transmission circuit including a positive terminal (101) of a signal line, a negative terminal (102) of a signal line, and a protective ground (103), characterized in that, Includes the following steps: A first variable capacitor network (220) is provided between the positive terminal (101) and the negative terminal (102) of the signal line to bypass differential mode interference signals; A second switched capacitor network (230) is provided between the positive terminal (101) of the signal line and the protective ground (103), and a third switched capacitor network (240) is provided between the negative terminal (102) of the signal line and the protective ground (103) to bypass common-mode interference signals; Real-time acquisition of noise characteristic parameters in low-voltage signal transmission loops; Based on the noise characteristic parameters, the equivalent capacitance of the first variable capacitor network (220), the second switched capacitor network (230), and the third switched capacitor network (240) are adjusted synchronously according to the preset dynamic mapping strategy. Among them, a bidirectional transient voltage suppression unit is connected in series between the second switched capacitor network (230) and the protective ground (103) and between the third switched capacitor network (240) and the protective ground (103). The bidirectional transient voltage suppression unit is configured to: present a high impedance state for low frequency voltages below its conduction threshold to block power frequency circulating current, and present a low impedance state for high frequency common mode interference or surge voltages above its conduction threshold to conduct bypass path and clamp protection.

2. The method according to claim 1, characterized in that: The dynamic mapping strategy includes: When the main peak frequency of the noise spectrum is detected to be higher than the first preset threshold, the equivalent capacitance of the first variable capacitor network (220), the second switched capacitor network (230) and the third switched capacitor network (240) are synchronously adjusted to the first capacitance range. When the main peak frequency of the noise spectrum is detected to be lower than the second preset threshold, the equivalent capacitance of the first variable capacitor network (220), the second switched capacitor network (230) and the third switched capacitor network (240) are synchronously adjusted to the second capacitance range. The value of the first capacity range is smaller than the value of the second capacity range.

3. The method according to claim 2, characterized in that, The first capacity range is 1000pF to 2200pF, and the second capacity range is 0.01μF to 0.1μF.

4. The method according to claim 1, characterized in that, The steps for real-time acquisition of noise characteristic parameters include: extracting the common-mode voltage fluctuation spectrum or differential-mode voltage fluctuation spectrum superimposed on the useful signal in the weak electrical signal transmission circuit through a high-impedance sampling unit.

5. The method according to claim 1, characterized in that, The bidirectional transient voltage suppression unit is a bidirectional transient voltage suppression diode.

6. The method according to claim 1, characterized in that, The weak current signal transmission circuit is a circuit that transmits any one of the following signals: 4-20mA current signal, 0-10V voltage signal, RS485 communication signal, thermocouple signal, or resistance temperature detector (RTD) signal.

7. An adaptive composite filtering device for mixed interference in weak current circuits, used to connect in a weak current signal transmission circuit including a positive terminal (101) of a signal line, a negative terminal (102) of a signal line, and a protective ground (103), characterized in that, include: The first variable capacitor network (220) is connected in parallel between the positive terminal (101) and the negative terminal (102) of the signal line; The second switched capacitor network (230) has one end connected to the positive terminal (101) of the signal line. The third switched capacitor network (240) has one end connected to the negative terminal of the signal line (102). The dynamic spectrum detection and control unit (210) has its input terminal coupled to the positive terminal (101) and / or the negative terminal (102) of the signal line, and is used to collect noise characteristic parameters in real time and output control signals according to the preset dynamic mapping strategy to adjust the equivalent capacitance of the first variable capacitor network (220), the second switched capacitor network (230) and the third switched capacitor network (240). The first bidirectional transient voltage suppression unit (250) is connected in series between the other end of the second switched capacitor network (230) and the protective ground (103); The second bidirectional transient voltage suppression unit (260) is connected in series between the other end of the third switched capacitor network (240) and the protective ground (103).

8. The apparatus according to claim 7, characterized in that, Both the first bidirectional transient voltage suppression unit (250) and the second bidirectional transient voltage suppression unit (260) are bidirectional transient voltage suppression diodes.

9. The apparatus according to claim 7, characterized in that, The first variable capacitor network (220) includes a voltage-controlled capacitor array composed of varactor diodes, or a switched capacitor array composed of multiple fixed capacitors and controllable switches.

10. The apparatus according to claim 7, characterized in that, The dynamic spectrum detection and control unit (210) includes an impedance transformation circuit, a bandpass filter circuit, an analog-to-digital converter circuit, and a microcontroller connected in sequence.

Images