Research Harmonic Detection Methods In Low-Voltage Power System

With the rapid development of modern industry and widespread application of power electronic devices, harmonic pollution is becoming more and more serious, which has a serious negative impact on the power quality as well as the normal operation of various electrical equipments. Harmonic detection is the main aspect of harmonic problem research. Real-time and accurate detection of harmonic quantity on power network is very necessary for preventing the harmonic harm and protecting safe operation of power network.This paper researched on harmonic current detection methods in low-voltage power system. The main contents are as follows:Firstly, the basic principles of harmonics, present situation and development trend of harmonic detection were studied. Various harmonic detection methods were briefly introduced and advantages and disadvantage of these methods were analyzed.Secondly, the instantaneous reactive power theory was analyzed. Then instantaneous reactive power theory based p-q harmonic current calculation mode and ip-iq harmonic current calculation mode were studied, which mainly focused on the impact of low-pass filter on the detection accuracy and speed. With theoretical analysis, spectrum characteristics of ip and iq were obtained and a reasonable choice of cut-off frequency was made. The application of wavelet analysis theory in harmonic detection was also studied for it is a representative for new harmonic detection methods.Thirdly, a three-phase thyristor based rectifier circuit of resistive and inductive load was realized as a harmonic source using POWER SYSTEM module in MATLAB/ SIMULINK. According to ip-iq harmonic current calculation mode, a harmonic detection model was established and simulation experiments were carried out in different firing angles. The experiments focused on the impact of low-pass filter on accuracy and speed of harmonic detection and the results verified the correctness of cut-off frequency derived from relative theory. The order and cut-off frequency of low-pass filter were chosen based on the simulation results. Wavelet analysis theory based harmonic detection methods were simulated and detection results with different wavelet decompositions were analyzed. Then application scope, advantages and disadvantages of instantaneous reactive power theory based harmonic detection method and wavelet analysis based harmonic detection method were discussed.Fourthly, hardware circuit for power harmonic detection system was designed with digital signal process chip TMS320LF2407A as the core, including signal collection circuit, generation circuit for square waveform signal and signal conditioning circuit. Then software for harmonic detection system was designed, including DSP initialization module, EV module, data collection module, digital filter module and so on.Finally, data collection card based harmonic detection system was designed using UA2011 data collection card from ADDATECH and VC++ 6.0 as development environment.