Supraharmonics Analysis of Modular Multilevel Converter and Long Cable System

Modular Multilevel Converter (MMC) has been applied in the demonstration project of flexible dc distribution network, whose output voltage spectrums is very wide. Field test cases show that when MMC is connected to the city distribution network through long cable, distributed capacitance of cable may cause the amplification of high frequency harmonics and even supraharmonics resonance is detected , especially in islanded power supply mode. This article establishes the mathematical model for converter-long cable-load system and the principle of high-frequency amplification is discussed. A characteristic equation to solve the resonance frequency is deduced by open source mathematical tools SciPy. The numerical calculation of resonance frequency at the grid-connection essentially agrees with the field test results.