Abstract: Taking medium voltage converter used in wind power plant as an example,the small signal models of Phase Locked Loops（PLL）and Current Control Loops（CCL）in the sub-synchronous frequency band of electrical inverters was established. The relationship between grid inductance,proportion and integration parameters of CCL,proportion and integration parameters of PLL,active power output,and grid Sub-Synchronous Oscillation（SSO）was analyzed by root locus methods. The research showed that the q axis current loop control can be characterized the series effect of resistance,inductance and capacitance when the electrical inverter is connected to the weak electrical power system,and the resistance will be negative resistance when proportions of the q axis CCL or PLL is small,which leads to SSO phenomenon of the regional power system without series compensation.