AbstractIn the early experiments of the LHD, the harmful third harmonic currents had been observed in the power line of the NIFS substation, when the fly-wheel motor generator (MG) for the neutral beam injection (NBI) reaccelerates between the plasma productions. The variable harmonic currents, which were estimated to be 1±6 and 1±12 ns, were induced through a Scherbius device. They extremely resonated to the circuit impedance of the power line. To stabilize electrical power in the site, following modifications were conducted: (1) reduction of the harmonic current from the Scherbius device; (2) reconnection of the power supplies of the SC coils and heating devices to suitable power feeders; (3) compensation of lagging reactive power of the MG. As a result, harmful harmonic currents were suppressed.