Research On Key Technical Issues Of Protection For Distribution Network With Renewable Energy

The distribution network is the main way to accept distributed power sources.The integartion of large-scale distributed power sources into the distribution network brings new challenges to the safe operation of the distribution network.A reliable protection scheme is mandatory to ensure that large-scale distributed power sources are safely integrated to the distribution network.This thesis selects three key technical topics related with the protection of new distribution network.First,islanding detection(antiislanding protection)is not only vital to the safety of personnels and electric facilities,but also directly affects the operation and control of distributed power sources in the distribution network.Existing passive islanding detection schemes need to be improved due to the non-detection zones and reliability under the power system disturbances.Second,the protection of the distribution network with large-scale distributed power sources needs to be applicable to the distribution network with the complex structure,short lines,and many un-measurable branches.Current differential protection has absolute selectivity and faster operation and it is considered as a main protection method for the distribution network in the future.It is meaningful to improve the economical efficiency of the current differential protection for the line with unmeasurable load branches and T-connected small power branches.Third,accurate fault location can faciliate a fast fault isolation and hence a shorter power interruption.In this thesis,μPMU is applied to the distribution network with large-scale distributed power access.Based on the high precision synchronized phasor measurement values,solutions are proposed for the above three key technical topics.The main contributions and research results are summarized as follows:1.It analyzes the impact of the system frequency deviation on the amplitude and phase angle measurement accuracy.A smaller frequency deviation can produce a relatively larger phase angle measurement error.This thesis presents a frequency estimation method based on positive sequence voltage.It does not require iterative calculations and it can direclty estimate power system frequency.It can ensure high frequency accuracy under conditions such as harmonic interference and three-phase imbalance.With the high precision real-time frequency measurement,the original voltage and current signals are resampled and then the high precision phasor measurements are calculated.In addition,this thesis suggests using low-power transformers(LPIT)to replace traditional electromagnetic transformers to ensure highprecision phasor measurement from the original signal source.Low-power transformers have both measurement-class accuracy and protection-class measurement ranges.Lowpower transformers are used together with μPMU as an integrated solution for both protection and measurement applications.(2)Based on the analysis of the operating characteristics of synchronous generators and inverter-type power sources under a sudden unplanned islanding condition,an islanding detection algorithm based on phase angle accumulation drift is proposed.The three-phase voltage phasors jump in the same direction is used as the starting criteria,to initiate the calculation of phae angle accumulation drift based on the real-time frequency measurement.When the system frequency becomes stable,the calculation of the phase angle accumulation drift is stopped.When phase angle accumulation drift is greater than the threshold,it is determined that islanding occurs.Compared with the conventional passive islanding detection methods,the islanding detection method based on phase angle accumulation drift has higher sensitivity,smaller non-detection zone,and faster detection time.Under system unbalance faults,such as three-phase faults,system heavy load switching and other system events,the proposed islanding detection method remains reliable.(3)This thesis presents an innovative idea to transform the line with T-connected unmeasurable load branch into a double-ended line with equivalent admittance connected at both ends,and the sum of the equivalent admittance at both ends is approximately equal to the admittance value of the T-connected load.The T-connected small power source is equivalent to the additional current sources at both ends of the line.The output current of the T-connected small power source and its location determine the equivalent current sources.The line with T-connected load and Tconnected power source is equivalent to a double-ended line.According to the real-time synchronous phasor of μPMU on both ends of the line,the real time parameters of the equivalent circuit are estimated and they are applied for current differential protection and fault location.(4)Based on the equivalent transformation of the line and its real-time equivalent parameters,this thesis applies the positive sequence current differential protection and the negative sequence current differential protection at the same time.The setting value of positive sequence current differential protection needs to consider the influence of unmeasured branches connected with the line;the sensitivity is relatively low and it is mainly to detect three phases faults;the setting value of negative sequence current differential protection does not need to consider the unmeasured branches and it has high sensitivity to unbalanced faults.In addition,this thesis prensets a fault location method using positive sequence superimposed values and negative sequence superimposed values respectively.When there is a fault in the main line,after the current differential protection trips,the fault zone is automatically isolated according to the fault location result.With the high-precision phasor and frequency measurement from μPMU,an islanding detection algorithm based on phase angle accumulation drift,positive and negative sequence current differential protections,positive and negative sequence fault location methods are proposed in this thesis.Their sensitivity and reliability are well verified by simulations.In addition,the equivalent transformation of complex lines in distribution network and related estimation of the equivalent parameters based onμPMU can simplify the theoretical analysis and it can facilitate the application of related protection technologies in complex distribution networks.