Resolving mode mixing in boiling water reactors instability analysis using variational mode decomposition

Resumen

In dynamic, complex and non-linear systems, the analysis of transients, instabilities or unusual events is fundamental, as well as the knowledge of the system's behaviour in the frequency space. What is usual in this type of study is to have discrete and digitally acquired data, due to a sampling process with a determined period or frequency. Extracting information from this data set is done by numerical methods, such as the Fourier Transform (FFT), the Hilbert-Huang Transform (HHT) or techniques from chaos mathematics (Attractors). In this way we have scarce data, with some loss of information, analyzed with numerical techniques that introduce, also, some dilution of the information. In particular, the techniques of analysis in the frequency space, by their very nature, blur, frequencies and bandwidths. This leads in some cases to the identification of unsuitable or very wide frequencies or intervals, which make the extracted information unusable. To this effect, we have studied the VMD technique that provides more real and representative frequencies of the system as well as narrower bandwidths that make possible better identification of phenomena and influences. This technique has been applied to the analysis of thermohydraulic instabilities in nuclear boiling water reactors (BWR). The cases had already been analyzed with traditional FFT and HHT techniques. The results of the application of the VMD are very good, improving over the previous ones the identification of main frequencies and their bandwidths. In this way, cleaner frequencies are obtained. So the VMD technique has better characteristics and improves the results of the previous techniques, posing as a very good tool for the analysis of transients or instabilities.