introduction
Electromagnetic compatibility (EMC) refers to the ability of a device or system to operate in its electromagnetic environment without posing an unacceptable electromagnetic disturbance to any object in the environment. The residual current protector must be satisfied as the power supply line protection device (below 400 V) at the end of the grid. EMC national standard GB/T17626.5-1999 requires that 3C certification can be put into grid operation. Figure 1 shows the block diagram of the intelligent residual current protector system designed with P87LPC767 microcontroller. The electromagnetic compatibility design is considered in circuit design, software design and PCB board design. The residual current protector is a kind of low-voltage electrical equipment. There is no high-frequency high-frequency circuit inside, and the electromagnetic radiation is weak. The electromagnetic disturbance generated by it has little influence on other equipments. This is not the focus of electromagnetic compatibility design. The focus of the residual current protector electromagnetic compatibility design is its ability to maintain stable operation when it is subjected to electromagnetic interference generated by other equipment, that is, anti-interference ability. The residual current protector is mainly from the power grid itself, mainly including unipolar surges (shocks) caused by sudden line breaks or lightning transient overvoltages, and signal parameters caused by lightning, ground faults or switching inductive devices. The generation of instantaneous disturbances is the focus of electromagnetic compatibility design and the difficulty of design. The electromagnetic compatibility design method of the intelligent residual current protector is introduced from various aspects below.
1 anti-interference design of single chip system
The residual current protector adopts P87LPC767 single-chip microcomputer, and the important work such as sampling detection, calculation, display, action criterion and protection action of residual current are all completed by single-chip microcomputer. The anti-interference ability of the single-chip system itself directly determines the anti-interference of the whole protector. ability. The clock signal, reset circuit, interrupt signal and sampling signal of the MCU itself are easily affected by electromagnetic interference, and it is very important to eliminate or suppress the influence of the electromagnetic interference signal on the MCU.
Specifically designed from the following aspects:
The working power of the single-chip microcomputer and the other circuit power supply of the system are designed separately to avoid the influence of other circuits on the working power of the single-chip microcomputer. The power supply design of the single-chip microcomputer has sufficient margin to prevent the fluctuation of the power supply from affecting the operation of the single-chip microcomputer. When designing the PCB board, connect the capacitor and the transient voltage suppressor (TVS) to the power supply pin of the microcontroller, as shown in Figure 2. The energy stored in the 100 μF electrolytic capacitor is released (lower) when the power supply fluctuates, keeping the power supply stable; the 0.1μF high-frequency capacitor can absorb the high-frequency interference on the power supply; the TVS absorbs the transient surge power, and the voltage between the two poles is clamped. Located at a predetermined value, keep the power supply of the microcontroller stable.
The reset circuit adopts the internal power-on reset circuit of P87LPC767 MCU to avoid the direct influence of the interference signal on the reset circuit. As long as the working power of the MCU is stable, there will be no malfunction caused by false reset. Interrupt and other I/O port capacitances are filtered to reduce the effects of interference.
In the system software design, the watchdog inside P87LPC767 is enabled to prevent the PC from being out of control due to interference, causing the program to fly and enter the "infinite loop". Software traps are set in non-program areas of the program memory space. When the operating system is out of control due to interference and enters the non-program area, the boot instructions are transferred to a program that specifically handles program errors, so that the program is put on the right track.
2 Elimination of high-frequency interference in the system————Sampling and monitoring method
The signal parameters caused by lightning, ground faults or switching inductive devices generate instantaneous disturbances, and the generated high-frequency interference signals mainly enter the system through the system incoming power. The anti-interference ability in this aspect is mainly verified by the "electric fast transient burst immunity test". According to the requirements of the test standards for low-voltage electrical products, the group pulse with a frequency of 2 500 Hz and an amplitude of 4 000 V is superimposed on the power supply input end, and the differential mode and common mode of the phase line and the ground line are respectively tested, and the positive polarity and negative polarity tests are performed. Each test time is 1 min, and the waveform of the group pulse is shown in Fig. 3. If the protector does not malfunction during the "electric fast transient burst immunity test", the anti-interference ability in this respect is qualified.
The system uses the "sampling monitoring" method to monitor the high-frequency interference on the power supply, and gives corresponding signs, which are processed by the software program to avoid malfunction of the protector. The detailed principle is as follows:
In order to ensure the real-time performance of the residual current measurement, the protector uses the digital sampling method to measure the residual current, converts the residual current signal into the waveform shown in Figure 4, and sends it to the A/D inside the P87LPC767 microcontroller for sampling measurement.
The digital sampling method has a digital filtering effect and suppresses the channel interference of the signal. Set 100 points per cycle and the sampling interval is generated by timer T0. The signal of a certain phase incoming power supply (such as phase A) of the system is stepped down and the shaping circuit is converted into a square wave signal as shown in FIG. As the trigger signal of the single-chip interrupt 0, the frequency of the power signal is about 50 Hz under normal conditions, and the number of times of sampling the residual current is 100 times during two interruptions.
When the power supply has a high-frequency interference signal, the converted square wave signal is also synchronously interfered, and the frequency is also superimposed with a high-frequency pulse, which will affect the interrupt, and the time of the two interrupt triggers is shortened, and the residual current is interrupted during the two interruptions. The number of samples will also be less than 100 times. By using the MCU interrupt to monitor the residual current sampling process, it can be judged whether the system is subjected to high-frequency interference, and the corresponding software processing can avoid the malfunction of the protector.
The software program for monitoring sampling is as follows:
The main program of the system judges bgrc1. If it is effective for interference software processing, if it is invalid, the measurement process is not subjected to high-frequency interference of the power supply, and normal calculation processing is performed.
3 Other anti-interference measures
Unipolar surges caused by open circuit or lightning transient overvoltage are also the focus of protector EMC design. A zinc oxide varistor is added between the live line and the neutral line of the power line. When the voltage applied across the varistor is lower than the nominal voltage, the resistance is almost infinite. After slightly exceeding the rated value, the resistance value drops sharply. The time is ns. The varistor can greatly attenuate the surge interference and reduce the impact on other circuits.
When designing the residual current protector PCB board, comprehensive consideration of various factors that may affect the performance of the protector, improve the PCB's ability to suppress interference. The high-voltage circuit concentrates one end of the edge of the board and maintains an appropriate distance from the weak current. The analog signal processing circuit and the digital processing circuit are separated. Since the A/D inside the single chip microcomputer is used, the two cannot be completely separated, and the analog ground and the digital ground are only shared at one point. The system power and ground lines are thickened, and the blank area is covered with copper. The mesh structure is used as part of the digital ground to reduce the interference of the analog signal to the digital processing circuit. The microcontroller system clock circuit is as close as possible to the chip pins, and maintains proper space with other devices and PCB traces to reduce the impact of high frequency radiation on the system.
Conclusion
Due to the correct and effective hardware and software measures taken, the protector successfully passed the product type test, obtained 3C certification, and operated reliably without any malfunction. The interrupt of the single-chip microcomputer is used to monitor the power supply interference of the system, and then the sampling process of the AC signal is monitored. Practice has proved that the combination of software and hardware is an effective way to eliminate high frequency interference in AC sampling systems.
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