Update about_yiheng_xie.md

Author: yxie20

_pages/descriptions/about_yiheng_xie.md

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 <b>PhD Student in Computing and Mathematical Sciences</b>
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-I am a PhD student at Caltech CMS, focusing on the modeling, analysis and optimization of energy systems. I am intersted in research questions with rigor and relevance. I am currently building a large-scale cyber-physical testbed at Caltech, including high-resolution synchrophasors (PMUs) metering and controllable inverter-based resources. 
+I am a PhD student at Caltech CMS, focusing on the modeling, analysis and optimization of energy systems. I am intersted in research questions with rigor and relevance. I spent the last two years building a high-resolution sensor network in California, dubbed "Digital Twin."