In this paper

in view of the issue of sub-synchronous oscillation of low voltage distribution gridcaused by the serial compensation device based on the inter-harmonic theory

the generation mechanismand transfer process of the sub-synchronous oscillation（SSO in short）of the low voltage system isanalyzed from both dimension of actually measured data and mechanism simulation

and it is verified thatthe SSO is caused by the series resonance of the equivalent capacitance of series compensation device andthe equivalent impedance

concluding the low voltage distribution grid and motor stator winding around at37.5 Hz to enlarge the inter-harmonics of air compressor.