Analysis and treatment of power supply failure cau

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Analysis and treatment of system power supply fault caused by frequency converter harmonic

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[CLC No.]tm921[document identification code] a Article No. (2002)

with the continuous improvement of the functions of the inverter itself, the AC speed regulation technology has made great progress. Under different working conditions, the AC drive system composed of AC asynchronous motor and frequency converter has the potential to replace the DC drive system. There have been many application cases of using frequency converter to complete the transformation design of AC drive system under different working conditions. Correspondingly, some new problems have been found in use. This paper analyzes the situation and treatment scheme of the system power failure caused by the series resonance of the control circuit caused by the high-order harmonic of the frequency converter during the normal operation of the frequency converter

the variable frequency speed regulation constant pressure water supply system is composed of frequency converter, PLC, PID regulator, supporting low-voltage electrical apparatus and pressure sensor, as shown in Figure 1. The pressure sensor in the system is responsible for detecting the system pipe pressure, transforming the pressure signal into a resistance signal as a feedback input to the PID regulator, After comparison with the given signal, the deviation value uses the optimized PI algorithm to output the control signal to control the output frequency of the frequency converter. When the experimental force is applied to more than 13kn points, a loud noise is heard to ensure the constant pipe pressure. The working power supply voltage of PLC, supporting low-voltage apparatus and PID regulator is AC220V. In this system, the user requires to be able to visually display the liquid level in the water storage tank. Therefore, the liquid level sensor and digital liquid level indicator are selected to measure and display the liquid level in the water tank. In practical work, it is found that when the frequency converter works normally, the digital liquid level display often shows false indication, garbled code, etc; When the frequency converter stops working, the system completely returns to normal. Obviously, this is caused by the interference of the high-order harmonic component of the frequency converter to the power supply. Generally, the most effective way is to add a power filter to the power supply of the control circuit. The electrical principle block diagram of the whole system is shown in Figure 1. After the commercial universal power filter was installed, the liquid level display system returned to normal, but then there were new problems. Fuse fu2 in the control circuit was frequently blown. Check the circuit after power failure, and no short-circuit point is found in the circuit. Through detailed observation on the site, it is found that during the gradual speed increase of the system, the short circuit fault of the products that are expected to replace the metal internal fixed support occurs frequently when the inverter operates between a certain frequency band. Moreover, after the load (motor) of the frequency converter is disconnected, the fault phenomenon still occurs frequently, and the fault disappears after the power filter is removed. Therefore, the filter was first checked. After disassembly, it was found that the common π type filter was used. The circuit is shown in Figure 2. It is found that there is no fault in the power filter itself. Further analysis of the working principle of the frequency converter shows that in the AC DC AC type frequency converter, electricity supplies power to the frequency converter through the three-phase rectifier bridge. The power supply current can be decomposed into a series of harmonic components including the fundamental wave and 6K ± 1 harmonic (k=1, 2, 3...) components by using Fourier series. The harmonic content decreases with the increase of the inductance of the incoming reactance and the DC filter reactance. According to relevant information, generally, the 5th harmonic, 7th harmonic, 11th harmonic and 13th harmonic still account for about 40%, 35%, 25% and 20% after adding reactor. Due to the influence of the frequency characteristics of the circuit parameters, the inductive reactance of the inductor is, the capacitive reactance of the capacitor is, and the equivalent complex impedance of the whole power filter is under the action of the Nth harmonic; Where is the equivalent inductive reactance of the filter and is the equivalent capacitive reactance of the filter. If the value of the equivalent reactance of the filter under the k-th harmonic is equal to the value of the equivalent capacitive reactance, we can see the equivalent complex impedance of the circuit at this frequency, that is, the circuit is in a resonant state. At this time, R is only the line resistance, and its value is very small. It can be approximately considered that the circuit is in a short-circuit state under the action of this harmonic. Based on the above analysis, it can be concluded that the short-circuit fault of the power supply is caused by the series resonance of the power filter caused by the high-order harmonic component output by the frequency converter. Generally, if the harmonic component causing resonance is not large enough, it should not cause short-circuit fault. However, when the effective value of the short-circuit current exceeds the protection range of the system fuse, it will cause short-circuit fault. It is not very appropriate to use this kind of power filter

after analyzing the cause of the fault, further anti-interference measures are taken for the control display system in view of the main sources of interference sources of the industrial control system, i.e. power supply interference, process channel interference and space electromagnetic interference. First of all, in view of the fault state of the above system, a large capacitor is used to replace the original power filter for filtering, which effectively solves the interference problem of the power supply system of the control system. Therefore, in the process of purchase, some brands with relatively high prices will be preferred. However, it must be emphasized that at the output side of the frequency converter, it is absolutely forbidden to use capacitors to absorb high-order harmonics, so as to prevent the inverter tube from being damaged due to large peak charging or discharging current at the moment when the inverter tube is turned on; Secondly, as the user requires that the relative position of the liquid level indicator and the frequency converter cannot be changed, the liquid level indicator is strictly shielded by metal, and the signal shielding wire and metal shielding layer are strictly isolated from the system work and safety grounding, and the spatial position of the signal wire and the power line is relatively vertical, The electromagnetic interference in space is effectively prevented

after taking the above measures, the operation of the whole system has returned to normal, indicating that the above measures are completely feasible. At the same time, further experiments found that in order to weaken the interference signal transmitted through the line, a small inductance can be introduced into the control circuit when the power supply of the control circuit does not adopt capacitor filtering. As shown in Figure 3, the resistance under power frequency is induced after the handling is completed

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