Asynchronous Sampling Based Power Quality Monitor

The establishment of a real-time PQ (power quality) monitoringsystem is one of the groundwork for China’s state grid operation center.A PQ monitor can be implemented in any segment of a grid system, andmeasures key indexes such as power supply reliability, harmonic wave,power consumption and transient disturbances in time. An overallnetwork of such monitors would help to realize closed loop qualitymanagement, grid safety risk analysis, and improve energy efficiency.The new generation of smart grid presents four new requirements onPQ monitors: real-time, comprehensive, extensible and databasecapability. Due to the defects on instrumental architecture or processingalgorithm, traditional PQ monitors can’t meet all these requirements. Thisthesis designs a kind of distributed monitoring system, which consists ofan ARM controlling unit and DSP measuring units. An extensiblemonitoring network can be flexibly established by the co-processing ofmulti-spot measuring units. It provides a comprehensive measurementover all the important power quality and power efficiency parameters.Great real-time performance is ensured by optimal design of the DSP system, with particular considerations to memory management, datatransmission and transaction control.The harmonic is one of the key PQ parameters, and it’s calculated byusing DFT method. When dealing with asynchronous sampling sequence,measurement accuracy is low on account of spectrum leakage andpicket-fence effect. Quasi-synchronous sampling method is employed tosolve this problem. The rationale of this method is to measure thefundamental frequency by band-pass filtering and the thresholdcomparison method, and then to reconstruct the original samplingsequence by interpolation. The reconstructed signal can be approximatelyregarded as the synchronous sampling signal, so the spectral leakage andpicket-fence effect are significantly eliminated. If a certain inter-harmonicwave exists, with its frequency falling into the transitional zone of thefilter, then the measurement accuarcy of fundamental frequency isaffected. This thesis presents a kind of successive approximation methodto improve the measurement precision under such inter-harmonicinterference. In the common power grid situation of frequency bias, noiseinterference and inter-harmonic interference, this method provides highermeasurement precisions than the tranditional windowed interpolated FFTmethod.Calibration experiments are conducted for this PQ monitor, and themeasurement precision of harmonic meets requirements under China’s national standard (GB/T19862-2005).