PURPOSE: To prevent the displacement of a tap due to the operation error of a complicated power system composed of a plurality of banks and many breakers by providing a specific single-parallel operation error switching detector and an operation controller.

CONSTITUTION: Single-parallel operation error switching detectors 62W78' which close or open secondary side breakers 9, 14, 19 and bus connection breakers 20, 21 of LRT at single or parallel operation time, detect whether the positions of switching at the mode changeover switch and bank changeover switch of each bank is correctly switched or not and warns and display the error, if error occurs, are provided in a power system A using transformers LRT 6, 11, 16 with a plurality of ON-load tank changers. Further, operation controllers 8'W61c which detect the closing states of LRT secondary breakers and bus connection breaker and taps without disconnecting the tank when the tap is displaced in parallel operation time are provided.