Study On Islanding Detection Methods For Active Distribution Networks

In recent years,with the over-consumption of non-renewable resources and the deteriorating trend of environmental pollution,distributed generators(DGs)using renewable clean energy such as solar and wind are widely connected to the distribution network and form the active distribution network.In the event of an unexpected grid blackout,the unplanned island consisting of DGs and loads may be formed,posing a significant risk to the safety of equipment and personnel.Therefore,the grid-connected DGs must be equipped with the islanding detection function.The existing detection methods are effective for single DG islanding detection,however,multi-DG islanding detection is prone to failure due to dilution effects resulting from the opposite disturbance direction of each DG.Therefore,this dissertation conducts a study on distributed photovoltaic(PV)inverters to analyze the causes of the dilution effect and investigate the measures to eliminate the dilution effect.This dissertation proposes two methods of multi-DG islanding detection based on inverter control,the main contributions are as follows:Firstly,this study analyzes the causes of dilution effects and proposes corresponding solutions by studying the principle of islanding detection.Utilizing active methods in multi-DG islanding detection,we analyze the cause of dilution effect according to the definition and detection standards for islanding.A simulation model based on MATLAB/Simulink is built for verification;subsequently,in order to eliminate the dilution effect,two measures based on inverter master/slave control and inverter peer-to-peer control are proposed by studying the control principle of the inverter.Secondly,a multi-DG islanding detection method based on inverter master/slave control is proposed by analyzing the relation between islanding reactive power balance and frequency.One DG is selected randomly and its inverter is set as the master inverter with fixed frequency control while the inverters of other DGs are set as the slave inverters with phase-locked loop control.To eliminate the dilution effect,the master inverter is then used to stabilize the islanding frequency and ensure each slave inverter resumes unit power factor operation.Moreover,to reduce the blind area of islanding detection,the reactive power disturbance method is used to break the reactive power balance of the islanding.The results show that the proposed method is effective in eliminating the dilution effect effectively and achieving blind zone-free islanding detection of single/multiple DG.Finally,aiming the problem that the master/slave control islanding detection method is prone to failing to eliminate the dilution effect due to the master inverter unexpectedly out of service,resulting in multi-DG islanding detection failure,a multi-DG islanding detection method is proposed based on inverter peer-to-peer control by analyzing the phase relation between the zero-sequence voltage and the three-phase voltage of the islanding.The method is capable of achieving isotropic disturbance of each DG by analyzing the control relationship between the DG voltage at the point of common coupling and the voltage at the inverter output side,and by investigating the control principle based on positive feedback disturbance of zero-sequence voltage.The control principle based on zero-sequence voltage positive feedback disturbance is investigated to achieve isotropic disturbance of each DG,and the effect of the island loading properties on the DG disturbance magnitude is investigated to analyze the islanding detection blind zone.The result show that this approach effectively mitigates the dilution effect across diverse load conditions and islanding scenarios,enabling single/multi-DG island detection without blind spots,and the experimental findings are consistent with theoretical analyses.