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Digital time relay anti-interference method

Digital time relay anti-interference method


With the continuous development of digital technology and related majors, relay protection technology has also developed greatly, such as the application of static relays in power systems. Among them, digital time relays have been widely used as basic components in various relay protection and In the automatic control loop, the required delay of the controlled equipment or circuit is obtained, and it is used to realize the selective cooperation of the main protection and the backup protection.


Digital time relays are used for relay protection, first used to replace electromagnetic and transistor type time relays. It can shorten the step of overcurrent protection, reduce the maintenance amount, and improve the correct rate of protection action. Safe and stable operation of main system and main equipment is protected. Because it has the characteristics of high accuracy, good stability, convenient setting, intuitive, no need to verify the setting value change, wide setting range, etc., it is welcomed by users. As a result, digital time relays are widely used in power systems.


However, in recent years, digital time relays have repeatedly malfunctioned in the power system, causing great losses to users. Misoperation causes such as poor system environment, use and maintenance problems, product quality problems, device damage, poor anti-interference performance, etc., but the problem that is difficult to handle is the poor anti-interference performance of digital time relays. In terms of anti-interference performance of time relay, I put forward my own opinion for reference.


1 Methods to improve anti-interference ability


1 major sources of interference


The interference of relays in power system operation is mainly electromagnetic interference. The sources are as follows:


(1) When the DC low-voltage circuit disconnects inductive loads (such as contactors, intermediate relays, etc.) or the contacts of electromagnetic current and voltage relays shake, fast transient pulse group radio waves are often generated;


(2) Inductive interference generated when high-voltage substations operate near high-voltage electrical equipment;


(3) Frequency-modulated electromagnetic waves and high-frequency electromagnetic radiation generated during arc discharges of portable walkie-talkies and adjacent or nearby equipment;


(4) Electromagnetic energy propagating through space in equipment such as pulse circuits, clock circuits, switching power supplies, transceivers, etc .;


(5) A discharge occurs when a charged operator touches the conductive parts of the equipment.


2 Propagation of electromagnetic interference


There are two main forms of electromagnetic interference propagation, namely conduction and radiation. Conduction is the current or voltage acting on the relay through the wire. Radiation acts on the relay in the form of an electromagnetic field through space. The main conduction path for digital time relays is the power line. Therefore, the main part of suppressing conducted interference is in the power supply part of the digital time relay.


3 Measures to improve anti-interference


According to the source of electromagnetic interference and interference methods and the working characteristics of digital time relays, the measures adopted to improve the anti-interference ability of digital time relays are mainly solved from the following aspects.


(1) Add EMI filter at the power input. An EMI filter is a low-pass filter, a multi-port network of passive components. It can not only attenuate the interference caused by the conduction propagation interference mode, but also have a significant suppression effect on the interference of the radiation interference mode. Such filters are particularly effective for low frequencies (20-100kHz). Then by selecting a suitable ferrite core, its suppression frequency range can be increased to 400MHz.


Due to the small size of the digital time relay and the limitation of the structure, the molded EMI filter is generally large and not suitable. The working frequency of the relay is not high, and the design and process are relatively low. At the same time, the cost can be reduced. Therefore, it is very feasible to directly design the EMI filter in the circuit.


The accessories are rigorously screened and can be selected to approximate the ideal state, but there are actually deviations.


The dielectric capacitance and inductance in the filter can be changed, and the coupling during the appropriate change can fully suppress the transient interference caused by line switches, contactors, and actuators.


(2) General anti-interference measures for digital circuits


①The clock frequency should be low when the work permits; ②Power lines and control lines must be decoupled to prevent external interference from entering; ③Decoupling capacitors should be added between the power and ground of each integrated circuit. Requires high-frequency performance of the capacitor; ④ Add decoupling capacitors on the signal lines that are not fast.


(3) Reasonably design the printed circuit board. ① The power and ground wires on the printed board should be wired in a "Well" shape to equalize the current and reduce the line resistance. ② When wiring, separate the high and low voltage wires and separate the AC and DC wires. ③ Input, The output line should not be close to electromagnetics such as clock generator and power line, and should not be close to fragile signal lines such as reset line and control line. ④ Cross wiring between adjacent boards; ⑤ Minimize the effective surrounding area of ​​power line routing, which can reduce Electromagnetic coupling; ⑥ The wiring of adjacent layers should be perpendicular to each other; 不要 Do not have branches in the wiring to prevent reflection and generate harmonics; ⑧ Connect the bypass capacitor correctly. When the digital circuit is working, the current changes suddenly, and a strong noise signal will be generated. The bypass capacitor should be correctly connected to the power line according to Figure 4; 集中 The ground point is concentrated.


(4) Reasonable wiring ① The input power line and ground wire should be as short as possible; ② the wiring between the boards or the connectors should be as short as possible. Separate the line from the line; ③ When wiring, the power line and the contact lead should be separated; ④ The positive and negative power lines should be twisted with each other to reduce common mode interference.


(5) Adopting new process ① Adopting surface mount technology and surface mount packaging technology, which can significantly reduce the stray parasitic capacitance and inductance due to the longer lead of the device, and simplify the design of the shield, so to a large extent Reduced electromagnetic interference and radio frequency interference. ② The use of multi-layer circuit boards from a 2-layer printed circuit board to a 4-layer printed circuit board can greatly improve the emission and immunity performance.