PROBLEM TO BE SOLVED: To execute higher harmonic measurement by injecting dispersedly an in-phase current synchronized by a non-integer-fold frequency of a system fundamental frequency from each low rank system to the high rank system side, regardless of the load state of each low rank system or the like. SOLUTION: One of plural injection devices 8a-8d is used as a standard device, and only the standard device 8a among the devices 8a-8d is driven before measurement, and an output current of an injection frequency by the device 8a is injected from a low rank system 6a to which the device 8a is connected, through a high rank system 1, to low rank systems 6b-6d to which residual devices 8b-8d are connected, and phases of the currents of injection frequencies of each system 6b-6d are detected. On measurement, the device 8a is driven, and simultaneously the devices 8b-8d are driven with output current phases shifted by 180 deg. from the detected phases of the currents of the injection frequencies before measurement at each low rank system 6b-6d, and the same phase currents synchronized by the injection frequencies based on the output current phase of the device 8a are injected simultaneously from the system 6b-6d to the high rank system side.
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