Research On Relay Protection Principle And Fault Location Method For Single-phase Grounding Faults In Distribution Networks

Relay protection and fault location are important measures to improve the safety,stability,and reliability of the power distribution networks.The rapidly-growing scales and complexity of distribution networks increase the risk of multiple faults.Moreover,underground cables have been more frequently utilized for urban distribution networks in large-and medium-sized cities in China,which necessitates the low-resistance grounding mode for neutral points.In addition,many distributed generators(DGs)are integrated into the distribution network.Considering the nonlinear-coupling between the DGs,the low-resistance grounding distribution network has become a scattered multisource complex power system,whose fault evolution mechanism and characteristics have also experienced profound changes.Therefore,the relay protection principle in distribution networks is confronting great challenges.Besides,the single-phase grounding fault is the most common fault type in distribution networks,and the fault characteristics are closely related to the grounding mode.Specifically,the fault location method of single-phase high-impedance faults in resonant grounding distribution networks is a major problem for utility companies and users.Consequently,this thesis focuses on the zero-sequence current protection for multiple single-phase grounding faults in low-resistance grounding distribution networks,as well as the fault location method of single-phase high-impedance faults in resonant grounding distribution networks.The main research contents and contributions are as follows:(1)An analysis method for multiple faults in low-resistance grounding distribution networks with inverter-interfaced distributed generators(IIDGs)is proposed based on the ideal transformer and the multiport network theory.Firstly,based on the nonlinear equivalent model of the IIDG with PQ control strategy,the generalized mathematical model of low-resistance grounding systems with IIDGs is established via the ideal transformer and the multiport network theory considering the loop current equations and the boundary conditions.Then,an iterative correction algorithm is proposed to calculate the fault current of IIDGs considering the scattered nonlinear couplings between them;with the proposed algorithm,the node voltages and branch currents across the network can be achieved under any multiple-fault conditions.It lays the foundation for the fault analysis and relay protection principle of multiple faults in distribution networks.Finally,the proposed fault analysis method is verified by the PSCAD/EMTDC simulation.(2)An adaptive zero-sequence current protection based on the current correction is proposed for low-resistance grounding distribution networks with IIDGs.Firstly,based on the proposed multiple fault analysis method,the distribution characteristics of the zero-sequence current during multiple faults in low-resistance grounding distribution networks with IIDGs are investigated.A zero-sequence current calculation method for low-resistance grounding distribution networks with IIDGs is proposed via the zero-sequence current compensation coefficient,which converts the zero-sequence current generated by multiple single-phase grounding faults on different feeders to its counterpart during a single fault.Subsequently,the adaptive zero-sequence current protection scheme is established based on the manufacturing message specification(MMS)service in the telecontrol communication IEC 61850,which adaptively sets the threshold current and modifies the zero-sequence current.Finally,the reliability and sensitivity of the protection scheme are verified by simulation results.(3)A fault location method of single-phase high-impedance ground faults in the resonant grounding network is proposed based on the phase characteristics of the zero-sequence transient current components.Firstly,the parallel resonance process during the single-phase high-impedance grounding fault in the resonant grounding system is analyzed.The distribution profile of the zero-sequence current on the underdamped state is investigated.Then,the concept of the relative phase of zero-sequence transient current components is proposed,which can be calculated with the measurement data from the feeder terminal unit(FTU).Subsequently,the relative phase difference between the upstream and downstream zero-sequence transient current components of the fault point is summarized.It is concluded that the relative phase difference at adjacent upstream and downstream FTUs is significant,which is used as the fault location criterion.In addition,a transition resistance estimation method is proposed to determine the threshold of the location criterion.The fault location scheme is as follows: each FTU reconstructs transient components of the zero-sequence current based on the extended Prony method,and uploads the data to the distribution automation(DA)master station.The DA master station identifies the fault position based on the relationship between the pre-set location criterion and measured relative phase differences.Finally,the correctness and practicability of the proposed method are experimentally verified via the RTDS and the FTU developed with a faulted location function.The results show that the scheme has good adaptability and strong tolerance for transition resistances.(4)A novel FTU-based fault location scheme for single-phase grounding faults in resonant grounding distribution networks with IIDGs is proposed based on the ratio between transient zero-sequence admittances(TZSAs).Firstly,the influence of IIDGs on the conventional fault location methods for resonant grounding systems is discussed.The parallel resonances during single-phase high-and low-impedance grounding faults are analyzed,respectively.Then,the concept of the TZSA is defined.The TZSA exhibits distinct distribution features on underdamped and overdamped states.It is concluded that the TZSAs from FTUs upstream of the fault point have the maximum value,while the TZSAs from FTUs downstream of the fault point are proximal to zero,regardless of underdamped or overdamped state,and the adjacent upstream and downstream TZSA values of the fault point always have the maximal difference.Based on this feature,the fault location criteria can be built.In the proposed location scheme,the ratio between the maximum values of the TZSA is calculated first.By comparing it with the location criteria,the DA master station locates the faulty feeder section.The accuracy of the fault characteristics and the proposed method are validated by experiments carried out with the RTDS and an FTU-based monitoring system.The proposed scheme has high adaptability and reliability to various faulty positions,fault resistances,arcing faults,fault inception angles,imbalances and the presence of IIDGs and noise.