Anti-interference method for digital time relay

With the continuous development of digital technology and related majors, relay protection technology has also made great progress, such as the application of static relays in power systems. Among them, digital time relays, as basic components, have been widely used in various relay protection and automatic control circuits, so that the action of the controlled equipment or circuit can obtain the required delay, and can be used to achieve the selective coordination of main protection and backup protection.

Digital time relays are used for relay protection, first used to replace electromagnetic and transistor time relays. It can shorten the level difference of overcurrent protection, reduce maintenance, and improve the correctness 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 check the setting value, and wide setting range, it is very popular among users. As a result, digital time relays are widely used in power systems.

However, in recent years, digital time relays have repeatedly malfunctioned in power systems, causing great losses to users. The reasons for malfunction include poor system environment, maintenance problems, product quality problems, component damage, poor anti-interference performance, etc., but the most difficult problem to deal with is the poor anti-interference performance of digital time relays. This article puts forward its own views on the anti-interference performance of digital time relays for reference.

1 Methods to improve anti-interference ability

1.1 Main sources of interference

The interference to relays in power system operation is mainly electromagnetic interference, which comes from the following sources:

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

(2) Inductive interference generated when high-voltage electrical equipment is operated near the high-voltage substation;

(3) Frequency modulation electromagnetic waves generated by mobile phones, portable walkie-talkies and adjacent or nearby equipment, and high-frequency electromagnetic radiation generated by arc discharge;

(4) Electromagnetic energy propagated through space by pulse circuits, clock circuits, switching power supplies, transceivers, etc. in the equipment;

(5) Discharge occurs when the operator with charge touches the conductive parts of the equipment.


1.2 Propagation mode of electromagnetic interference

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

1.3 Measures to improve anti-interference

According to the source and interference mode of electromagnetic interference 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 to the power input end. EMI filter is a low-pass filter, a multi-port network composed of passive components. It can not only attenuate the interference caused by the conduction propagation interference mode, but also has a significant inhibitory effect on the interference of the radiation interference mode. Such filters are particularly effective for low frequencies (20-100kHz). By selecting a suitable ferrite material core, its suppression frequency range can be increased to 400MHz.

Due to the small size of digital time relays and the limitations of the structure, the molded EMI filter is generally large in size and not applicable.

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 also be reduced. Therefore, it is very feasible to directly design an EMI filter in the circuit.

After strict screening, the accessories can be selected to be close to the ideal state, but there are actually deviations.

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

(2) General measures for digital circuit anti-interference

① The clock frequency should be selected as the lowest under the conditions allowed by the work; ② The power line and control line must be decoupled to prevent external interference from entering; ③ A decoupling capacitor must be added between the power supply and ground of each integrated circuit. The capacitor is required to have good high-frequency performance; ④ Add a decoupling capacitor to the signal line with slow speed.

(3) Reasonable design of printed circuit boards ① The power and ground wires on the printed board should be laid out in a "well" shape to balance the current and reduce the line resistance; ② When wiring, separate the high and low voltage lines, and separate the AC and DC lines; ③ The input and output lines should not be close to electromagnetic heat lines such as clock generators and power lines, and should not be close to fragile signal lines such as reset lines and control lines; ④ Cross wiring between adjacent boards; ⑤ Minimize the effective enclosing area of ​​the power line as much as possible, so as to reduce electromagnetic coupling; ⑥ The wiring of adjacent layers should be perpendicular to each other; ⑦ The wiring should not have branches to prevent reflection and harmonics; ⑧ Correctly connect the bypass capacitor. When the digital circuit is working, the current mutation is large, which will generate a strong noise signal. The bypass capacitor should be correctly connected to the power line according to Figure 4; ⑨ The grounding point is concentrated.

(4) Reasonable wiring ① The input power line and the ground line should be as short as possible; ② The connection between boards or the connector connection should be as short as possible. And the lines are separated; ③ When wiring, the power line and the contact lead line should be separated; ④ The positive and negative power lines should be twisted together to reduce common mode interference.

(5) Adopting new technology ① Using mounting technology Using surface mounting packaging technology can significantly reduce the stray parasitic capacitance and inductance caused by the long leads of the device, simplify the shielding design, and thus greatly reduce electromagnetic interference and radio frequency interference. ② Using multi-layer circuit boards from 2-layer printed circuit boards to 4-layer printed circuit boards can greatly improve the emission and anti-interference performance.

2 Conclusion The

above analysis of the anti-interference problem of digital time relays. After practical application, the interference problem existing in the previous digital time relays has been solved, and the action reliability has been greatly improved. It has also been confirmed in the field application of a certain power system.