MMC is the core equipment of energy conversion of flexible DC distribution grid and AC power grid.The quality of converter under normal operation is verified by the power quality test at the AC interconnection point of the converter.However

in case of weak grid operation condition at isolated island mode

the AC interconnection point through long distance cable interconnection has high order harmonic resonance phenomena

which degrades seriously voltage waveform.In this paper

small coefficient mathematical model for converter-long cable-load system is set up and the intrinsic mechanism of high frequency harmonic transmission is studied.The Numerical analysis is performed by such Python open source software package as Numpy and Scipy etc.It is verified that the necessary prerequisites for the occurrence of high frequency resonance is the weak power grid.The characteristic equation of long cable and weak grid of inherent resonance frequency is deduced and solved.The rule that the increase resonance frequency of the impedance of bridge arm reactor reduces with the increase of cable length.The theoretical calculation result and the actual measurement result are basically identical.The MMC inverter with long cable interconnection is suggested and high frequency filter is mounted on opposite side of cable to suppress harmonic amplification of weak grid in operation.