Research On Adaptation Of AC Current Differential Protection At The Inverter Side In HVDC/AC Interconnected System

The imbalanced distribution of energy and load in China makes high voltage direct current(HVDC)power transmission become increasingly important.However,the infeed of HVDC also leads to malfunction of AC relay protection,which causes damage to existing power grid.Therefore,it is important to study the interaction between DC and AC systems after fault.Based on which,the adaptation analysis and promotion of AC relay protection in HVDC/AC interconnected power grid is studied in this paper.First of all,this paper studies the influence of AC system fault on HVDC commutation process and concludes the regulation between AC fault time and HVDC commutation failure valves.For AC faults which do not lead to commutation failure,this paper constructs the current switching function model of inverter suitable for unsymmetrical situations of HVDC/AC system and do not needs iterative computations.For AC faults which lead to commutation failures,this paper calculates the 50 Hz component of current switching function through theoretical waves,which makes the application of switching function theory more complete.Secondly,this paper proposes an inverter side DC current calculation method based on the control strategy.By combining this method with current switching function results,this paper calculates the equivalent inverter side AC current and points out that the current limiting of control system and the influence of HVDC commutation failure make the equivalent AC current decrease after fault.This makes AC system appear nonlinear characteristics and new calculation method needs to be proposed.Finally,this paper points out that the adaptation of current differential protection based on fault component is severely affected by the sudden decrease of equivalent AC current.Therefore,a self-adaptation differential protection method based on current decreasing detection is proposed.PSCAD/EMTDC simulation results prove that the new method is unaffected by commutation failure and is suitable for various fault scenarios and operation situations.This paper provides theoretical basis for further study on HVDC/AC interconnected power grid and the adaptation analysis of other relay protections.