How to improve the anti-interference ability of di

2022-06-21
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How to improve the anti-interference ability of digital time relay

with the continuous development of digital technology and related disciplines, the digital time relay, as a basic component, is used for relay protection. First, it is used to replace the electromagnetic type and crystal. This method is based on the tube type time relay when the test piece is in the stretched state and attacked by ozone. It can shorten the level difference of overcurrent protection, reduce the amount of maintenance, and improve the correct rate of protection action. It protects the safe and stable operation of the main system and main equipment. Because it has the characteristics of high precision, good stability, convenient and intuitive setting, no need to verify the time-varying setting value when changing 6, and wide setting range, it is deeply welcomed by users. Therefore, digital time relay is widely used in power system. However, due to the poor system environment, use and maintenance problems, product quality problems, device damage, poor anti-interference performance and other reasons, the digital time relay has misoperated many times in the power system, causing great losses to users. The most fundamental reason is the poor anti-interference performance of digital time relay. In view of this situation, this paper puts forward some views, hoping to be helpful

1 improve anti-interference ability

1.1 main sources of interference

in order to solve the problem of anti-interference, we must first understand the source of interference. Generally, the interference to the relay in the operation of the power system is mainly electromagnetic interference, which comes from the following sources:

(1) inductive interference generated when the high-voltage substation is close to the operation of high-voltage electrical equipment

(2) discharge occurs when charged operators touch the conductive parts of the equipment

(3) electromagnetic energy transmitted through space in equipment such as pulse circuit, clock circuit, switching power supply and transceiver

(4) high frequency electromagnetic radiation generated by frequency modulation electromagnetic wave and arc discharge of mobile, portable walkie talkie and adjacent or nearby equipment

(5) when the DC low-voltage circuit disconnects the inductive load (such as contactor, intermediate relay, etc.) or the electromagnetic current and voltage relay contacts jitter, it often produces fast transient pulse group radio waves

1.2 propagation modes of electromagnetic interference

the main propagation modes of electromagnetic interference include radiation and conduction. Radiation acts on the relay in the form of electromagnetic field through space. Conduction is acting on a relay in the form of current or voltage through a wire. The power line of the digital time relay is the main conduction path, so the main part to suppress the conducted interference is the power part of the digital time relay

1.3 measures to improve anti-interference

according to the source and mode of electromagnetic interference and the working characteristics of digital time relay, the measures to improve the anti-interference ability of digital time relay are mainly solved from the following aspects

(1) general anti-interference measures for digital circuits

① the lowest clock frequency shall be selected under the working conditions; ② The power line and control line must be decoupled to prevent external interference from entering; ③ Decoupling capacitor shall be added between power supply and ground of each integrated circuit. High frequency performance of capacitor is required; ④ Add decoupling capacitor to the signal line with low speed

(2) reasonable wiring ① input power line and ground wire shall be as short as possible; ② The wiring or connector wiring between boards shall be as short as possible. And the lines are separated from each other; ③ During wiring, the power line and contact outgoing line shall be separated; ④ Positive and negative power lines shall be twisted to reduce common mode interference

(4) reasonably design the printed circuit board ① the power supply and ground wire on the printed circuit board shall be well wired to balance the current and reduce the line resistance; ② During wiring, high and low voltage lines shall be separated, and AC and DC lines shall be separated; ③ The input and output lines shall not be close to the clock generator, power line and other electromagnets, and shall not be close to the fragile signal lines such as reset line and control line; ④ Cross wiring between adjacent boards; ⑤ Try to reduce the effective surrounding area of power line routing, so as to reduce electromagnetic coupling; ⑥ The wiring of adjacent layers shall be perpendicular to each other; ⑦ The wiring shall not have branches to prevent reflection and harmonic generation; ⑧ Connect the bypass capacitor correctly. When the digital circuit is working, the sudden change of current is large, which will produce a strong noise signal. The bypass capacitor should be correctly connected to the power line according to figure 4; ⑨ The grounding points are centralized

(5) add EMI filter at the power input. EMI filter is a kind of low-pass filter, which is composed of passive components. It can not only attenuate the interference caused by conducted propagation interference, but also significantly suppress the interference caused by radiated interference. Such a filter is particularly effective for low frequencies (20 100kHz). By selecting the appropriate ferrite material core, its suppression frequency range can be increased to 400MHz

conclusion

the anti-interference problem of digital time relay is analyzed above. Through practical application, the interference problem of the previous digital time relay has been solved, and the operation reliability has been greatly improved. The field application in a power system has also been confirmed

due to the small volume of the digital time relay and the limitation of the structure, the formed EMI filter is generally large in volume and not applicable

however, the operating frequency of the relay is not high, and the design and process requirements are relatively low. At the same time, the cost can be reduced. Therefore, it is very feasible to directly design EMI filter in the circuit

after strict screening, the accessories are selected to be close to the ideal state, but there is actually a deviation in winning the recognition of users

the dielectric capacitance and inductance in the filter can be changed. The coupling during appropriate change can fully suppress the transient interference caused by line switch, contactor, actuator and contact jitter

(3) adopting new process ① adopting mounting technology and surface mounting and sealing technology can significantly reduce the stray parasitic capacitance and inductance caused by the long lead of the device, simplify the shielding design, and reduce electromagnetic interference and RF interference to a great extent. ② The transmission and immunity performance can be greatly improved by changing the multilayer circuit board from 2-layer printed circuit board to 4-layer printed circuit board

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