PROBLEM TO BE SOLVED: To inject dispersedly an in-phase current or a voltage synchronized by a non-integer-fold frequency of a system fundamental frequency to the measuring side, namely, to the high rank system side from injection devices of each low rank system, regardless of the load state of each low rank system. SOLUTION: Before measurement, each injection device 6a-6c is respectively operated independently, and currents of injection frequencies flowing from injection points Pa-Pc to the load side are measured on each low rank system 5a-5c respectively, to obtain load impedances of the low rank systems 5a-5c, and the phase change based on the load impedances of the currents or the voltages of the injection frequencies injected from the injection points Pa-Pc to the high rank system side. On measurement, output phases by each injection device 6a-6c are shifted and corrected so as to counterbalance phase changes based on the load impedances of each system 5a-5c, and the same phase currents or the voltages synchronized by the injection frequencies are injected simultaneously from the injection points Pa-Pc of the systems 5a-5c to the high rank system 1 side by the correction.
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