High growth in electricity demand during the period of Japan's bubble economy resulted in the construction of many power transmission facilities, including some loop systems. In some loop systems, there is a possibility of considerable power flow differences between heavy and light loads. Significant changes in the power system, such as from damage due to a natural disaster, would likely result in even larger differences and ineffective use of the loop system. Phase shifting transformers (PSTs) are highly efficient equipment for solving such power flow problems and some studies of operation methods using active power flow sensitivity have been carried out. Calculating active power flow sensitivity, however, requires using the DC method. Since doing so requires the fulfillment of several assumptions, errors may appear in the calculation results. This paper assumes protracted significant local loss of generation capability (about 20% of total capacity), and examines the resulting load changes in the loop system. We then propose use of PSTs to alleviate load differences among transmission lines, allowing more effective use of the loop system. Specifically, we propose the concept of active power flow sensitivity (APFS) and active power flow sensitivity 2 (APFS2). Installing PSTs at optimal locations and operating the PSTs with proper phase angles determined by a branch and bound method, we mitigate loading differences between heavily and lightly loaded transmission lines. The validity of the proposed method is confirmed by using a modified IEEJ EAST 30 model.
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