In the non-solidly grounded distribution network

poor line transposition

open-phase power supply

Petersen coil's voltage resonance and other factors lead to the asymmetry of three-phase voltage

giving rise to a zero-sequence voltage in the system harmful to safe operation of the power grid. This paper analyzes the generation mechanism of asymmetrical voltage in a non-solidly grounded system

studies the relationship between the asymmetrical voltage and the current injected into the system according to the exchange characteristic of voltage and current

and proposes a novel active control principle to eliminate asymmetrical voltage by injecting a current into the system. Based on the active compensation device

this paper also proposes a practical closed loop control method to eliminate asymmetrical voltage adapted to the dynamic change of system structures by itself. This method starts with an arbitrary initial injection current. Firstly

fix the injection current amplitude and change its phase to find the phase that makes the asymmetrical voltage minimum. Secondly

fix the phase found in step one and change its amplitude to find the amplitude that makes the asymmetrical voltage globally minimum

or approach zero. The simulation has verified the proposed principle is able to control the asymmetrical voltage flexibly and effectively while maintaining the symmetry of three-phase voltage. This work is supported by National Natural Science Foundation of China (No. 51177096

No. 51477184) and National High Technology Research and Development Program of China (863 Program) (No. 2012AA050213).