Asynchronous Sampling Based Power Quality Monitor

In recent years, grid power pollution is getting more and more serious as the increase of asymmetric, shocking and nonlinear load. Meanwhile, a lot of precise electronic equipments are widely used, which demand a better power quality. Grid accidents caused by power quality problems happen all the time. To monitor power quality effectively is of vital importance to grid power quality improvement and accident responsibility appraisal.For most traditional power quality monitoring system, it usually has a single function, cannot monitor in real time and do not have the function of measuring transient parameters. According to the national standards and other international standards, concepts, algorithms and GB limits of power parameters are well presented. Causes, hazards and improvement methods of power quality problems are analyzed as well. Using DSP+ARM as the core, the system in this paper is based on asynchronous sampling. It can accurately measure RMS of voltage and current, frequency, three-phase unbalanced dgrees, voltage fluctuation and flicker, transient parameters and so on. As harmonic parameters are hard to get in the situation of asynchronous sampling, a quasi-synchronous algorithm is proposed, which solves the problem that PLL based synchronous sampling is prone to loss of lock when the nonstationarity of the signal gets to some extent as well.The system adopts the idea of module design. It first extracts grid signal from by transducers; adjust the signal to a acceptable degree through an amplifier; a independent transient detection module is designed at this level as the transients could be much faster than available ADC, which makes ADC unsuitable for transient detection; then both parameters are collected and calculated by the data processing mudule which uses DSP as the core; finally, all the information are collected and analysed statistically by the ARM module which also has the function of remote transmission through Ethernet and storing the information locally for about a week.This power quality monitor has already passed the test of national standard (GB/T 19862-2005).