Electromagnetic compatibility analysis of single-chip microcomputer system for intelligent frame-type circuit breaker

Electromagnetic compatibility analysis of single-chip computer system for intelligent frame-type circuit breaker

1. Brief Introduction

With the continuous development and improvement of single-chip microcomputer technology, single-chip microcomputers have been widely used in all walks of life, and their application degree and application level have gradually become an important indicator of the technical level of an industry or a department. As the main switch of the low-voltage power distribution system, the frame circuit breaker has also broken through the traditional structural mode and adopted single-chip microcomputer technology in its protection and information processing unit. Typical products include Schneider's M series, GE's S series, Mitsubishi's AE-SS series, and my country's DW45 series. As we all know, frame circuit breakers play a vital role in the safe and reliable operation of low-voltage power distribution systems, and the electromagnetic compatibility of their single-chip microcomputer systems is the core issue of their safe, reliable and stable operation. The following analyzes the purpose of electromagnetic compatibility testing, the harm of electromagnetic interference noise to single-chip microcomputer systems, and the anti-interference of the hardware and software of the single-chip microcomputer itself, in order to further improve the electromagnetic compatibility of single-chip microcomputer systems.

2. Purpose of electromagnetic compatibility test

The so-called electromagnetic compatibility refers to the performance of a device or system that, when actually operating in its predetermined electromagnetic environment, is neither affected by the surrounding electromagnetic environment nor affects the surrounding environment, and does not experience performance deterioration or malfunction, but can work normally in accordance with technical requirements.

In order to ensure that the intelligent frame type circuit breaker has excellent electromagnetic compatibility, the following electromagnetic compatibility tests are generally carried out.

1. The purpose of the low-frequency disturbance immunity test for power lines is to verify the anti-interference ability of the power system when harmonics, current sags, and current interruptions occur.

2. The purpose of the electrostatic discharge immunity test is to verify the anti-interference ability when the electrostatic charge carried by the human body or objects discharges to the accessible parts of the circuit breaker.

3. The electrical fast transient pulse group immunity test is to verify the anti-interference ability when the signal parameters are caused by lightning, ground fault or switching inductive circuits. This test is a pulse group test composed of many fast transient pulses coupled to the power line, control line and signal line.

4. The purpose of the surge (impact) immunity test is to verify the anti-interference ability of the unipolar surge (impact) caused by circuit breaking or lightning transient overvoltage. This interference is very harmful to the intelligent frame circuit breaker with the single-chip system as the protection core, and in severe cases it will lose its protection function.

5. The purpose of the radio frequency electromagnetic field radiation immunity test is to verify the anti-interference ability of the circuit breaker in various electromagnetic radiation environments such as handheld radio transceivers, radio broadcasts, vehicle-mounted radio transmitters, various industrial electromagnetic sources, etc. For this kind of electromagnetic interference from space, the method of adding metal shielding around the single-chip microcomputer system can achieve better results.

3. Impact of Electromagnetic Interference on MCU System

The electromagnetic interference mentioned above has a great influence on the clock signal, interrupt signal, sampling signal, etc. of the microcontroller itself. In addition, when the electromagnetic interference noise exceeds a certain threshold, it will cause the microcontroller system to fail or even crash. Therefore, this article only analyzes the anti-interference of the microcontroller system itself.

There are generally the following situations when the MCU system is abnormal: the MCU is abnormal for a moment, but can be corrected automatically; after the MCU is abnormal, an interrupt or reset signal is used to restore it to work; after the MCU is abnormal, operator intervention or system reset is required; the MCU is abnormal and causes component damage, data loss, data errors, etc.

4. Analysis of hardware anti-interference circuit of single-chip microcomputer system

According to different functions, there are many anti-interference circuits and methods for single-chip microcomputer systems, such as shell shielding technology, power grid filtering technology, system grounding technology, reasonable wiring, etc. This article only analyzes and discusses the fast reset circuit and watchdog circuit after the single-chip microcomputer system is interfered.

1. Fast and reliable reset circuit

Figure 1 is a simple RC power-on reset circuit. During the charging process of the RC circuit, a positive pulse appears at the RESET terminal, thereby resetting the microcontroller. In this reset circuit, interference is easily connected to the reset terminal. Although it will not cause an incorrect reset of the microcontroller under normal circumstances, it will cause an incorrect reset of its internal registers, causing system malfunction.

The reset signal is one of the most sensitive signals of the microcontroller. In order to make the microcontroller system work safely and reliably, the reset circuit has functions such as fast power-on reset and power-off reset. The reset circuit composed of the dedicated reset chip MC33064/MC33164 produced by Motorola and similar products produced by other companies can be used to improve the reliability of reset after the system is disturbed, as shown in Figure 2.

2. Watchdog circuit

In order to improve the anti-interference ability of the circuit breaker microcontroller system, it is best to directly select a microcontroller with a watchdog circuit. It can select hardware watchdog or software watchdog through user programming, and perform system reset operation on the watchdog counter through user program.

For a microcontroller without an internal watchdog circuit, an external watchdog circuit can be designed, as shown in Figure 3.

In the figure, 555 is connected to a multivibrator, and the count is hexadecimal. When the 8th pulse is counted, the Q terminal outputs a high level. An I/O port of the microcontroller is used to control the counter reset terminal. As long as the pulse time of each reset is less than 8 pulse cycles, the Q terminal will maintain a low level. When the microcontroller is abnormal, the counter cannot be reset within 8 pulses, and the Q terminal outputs a high level to reset the microcontroller system.

5. Software Anti-interference Measures for Circuit Breaker MCU System

1. Refresh I/O circuit in MCU

In the microcontroller structure, the I/O circuit is generally close to the edge of the die, and external electromagnetic noise can easily affect its logic circuit. Therefore, on the one hand, hardware measures should be taken, such as adding capacitors to the I/O ports for filtering. On the other hand, the registers in the I/O should be refreshed regularly in the user program to reduce malfunctions caused by electromagnetic interference.

2. Use digital filtering technology

The signals from the sensor are often mixed with interference signals of various frequencies. The use of RC low-pass filters at the signal end can suppress high-frequency interference signals, but the filtering effect on low-frequency interference signals is poor. Digital filtering technology can filter extremely low-frequency interference signals from the power grid. There are many digital filtering methods. The following only uses the root mean square method to process and analyze the input current and voltage digital signals.

The RMS method is based on the definition of the effective value of a continuous periodic AC signal in electrical principles. It discretizes the continuous signal and replaces the continuous integration with numerical integration, thereby deriving the relationship between the effective value and the sampling value.

Formula for calculating effective value

Use the detection circuit to uniformly sample the AC current and AC voltage signals, that is, divide a cycle into n equal parts, then the sampling period △t=T/n

at this time

When this algorithm is applied to continuous periodic AC signals, it has high calculation accuracy and strong ability to resist waveform distortion. It can continuously sample, reduce the malfunction of the single-chip system, and improve the system's resistance to electromagnetic interference. The sampling process is shown in the figure below.

3. Set up the self-diagnosis program

In order to prevent the data in the RAM area of ​​the single-chip system from being destroyed by electromagnetic interference, a check mark can be set in the RAM area, and the self-diagnosis program can be used to check these marks regularly. If the mark is normal, the program will continue to execute. If the mark is abnormal, the program will be imported into the initialization address to reset the system. Of course, this self-diagnosis can only diagnose the data in the key RAM, which is also a very basic self-diagnosis method.

VI. Conclusion

With the application of single-chip microcomputer technology in the field of low-voltage electrical appliances, the electromagnetic compatibility of intelligent frame-type circuit breakers should be given sufficient attention. Its electromagnetic compatibility should be improved from both hardware and software aspects. According to the specific application sites of the circuit breakers, an organic combination of hardware and software should be adopted to achieve twice the result with half the effort and effectively ensure the safe and reliable operation of the low-voltage distribution system.