With the background and basic principle of the electrical spring(ES)described

the modulability of ES is analyzed at length by the vector diagram method

and the operation range of the line voltage under which ES can work is investigated. Besides

the malfunction mechanism is analyzed

and a new computational method for optimizing the specified value of critical load voltage is found. When the line voltage changes in the range

the critical load voltage can be stabilized on the standard value by ES

but when changes are beyond the range

the ES will break down

and the critical load voltage will deviate from the standard state

or even the system can become unstable. Given the permissible fluctuation range of the critical load voltage

the expanded safety regulation domain of the ES system can be obtained by optimizing the specified value of the critical load voltage. Finally

the analysis results are validated by simulation and experiment results.