The Research Of Unbalanced Voltage Control In Non-solidly Grounded System Based On Active Compensation Techniques

In the non-solidly grounded systems,three-phase voltage unbalance can be caused or aggravated by poor line transpositions,open phase operations,and Petersen coil's voltage resonances.The appearance of the zero-sequence voltage in the system endangers the safe operation of the grid.But traditional methods have different disadvantages,such as high difficulty in operation,increasing equipment investments and poor adaptation in system with dynamic changes.Their effects are not ideal.The appearance of active compensation techniques provides a novel and more effective way for unbalanced voltage control.This paper analyzed the generation mechanism of unbalanced voltage in the non-solidly grounded systems.Considering the condition of only considering unbalanced three-phase admittance to ground,as well as considering both unbalanced three-phase impedance and three-phase admittance to ground,the relationship between the unbalanced voltage and the current injected by active compensation device was given for either case,and a novel principle of unbalanced voltage control based on injection current was proposed.The injection currents to control unbalanced voltage to zero are equal to the neutral current when this system neutral point is supposed to be solidly grounded in isolated neutral system and resonant grounded system.Compared with ignoring the impedance of lines,the injection current to control unbalanced voltage to zero adds an additional current caused by line drop after considering the impedance of lines.In accordance with the variation of unbalanced voltage along with injection current's amplitude and phase studied in this paper,three practical closed-loop unbalanced voltage control methods based on active compensation technology were given to adapt to frequent system structure changes,such as two-point method(calculate injection current by two unbalanced voltages corresponding to two different unbalance conditions of system),tracking method(regulate phase and amplitude of injection current step by step in turn),and synthetic method(injection current calculated by two-point method is used as the initial value of tracking method).They can control unbalanced voltage to any target value.Synthetic method combines all advantages of the two-point method and the tracking method,and avoids their disadvantages.Its control performance is best.The simulation verified the correctness of the mechanism of unbalanced voltage,the principle of unbalanced voltage control based on injection current,and the methods proposed in this paper.The superiority of these methods compared with traditional methods was also verified.Compared with traditional methods,the three control methods based on injection current have advantages of little investments,flexible control,high accuracy,no need to measure the three-phase parameters of lines,and adapting to the dynamic change of system structures automatically.They could control the unbalanced voltage flexibly and effectively,and keep the balance of three-phase voltage.