Research On Weak Fault Detection Method Of Distribution Network Based On Dq Transform

With the development of science and technology,users’ demand for power supply reliability is gradually becoming higher,and reducing the frequency of fault outages is one of the main measures to improve the reliability of power supply in distribution networks.China’s distribution network relay protection mainly adopts the three-stage current protection,when the fault current is small or the duration is short,the relay protection device will not operate.The fault that cannot cause the relay protection to act is collectively referred to as a weak fault in the distribution network.As the weak fault occurs in the system,the protection device fails to detect it in time and cannot be dealt with in time.As the fault level on the power supply line increases,it eventually leads to a permanent fault and causes unscheduled power outage.If the weak faults in the distribution network system can be detected accurately and the hidden problems can be eliminated in time,it is of great significance to improve the reliability of the distribution network power supply and prevent permanent faults.In this dissertation,the weak faults of different grounding methods of distribution network neutral point are taken as the research object,and the characteristics of single-phase ground fault and early fault of cable in resonant grounding system are mainly analyzed.The dq-transform detection algorithm is selected to detect the fault characteristics and the deficiencies existing in the traditional detection algorithm are improved to meet the fault detection requirements.The main work performed is as follows.(1)Proposed a dq-transform electrical eigenvolume detection algorithm for fault detectionThe conventional dq-transform electrical eigenvolume detection algorithm can only detect the steady-state eigenvolume in the signal,but not the transient eigenvolume in the fault signal.For this reason,this dissertation proposes a dq-transform detection algorithm for fault eigenvolumes.For the detection of steady-state eigenvolumes,the Delay Sampling Period Filter(DSPF)is used to effectively solve the problem of detection accuracy and response speed of the detection algorithm in sudden change signals.For the detection of transient eigenvolume,the Decaying Direct Current(DDC)component of the fault transient signal is approximated by the Delay Sampling Period method;meanwhile,the combination of DSPF can effectively reduce the steady-state error in the amplitude and phase calculation after the fault,which solves the problem that the traditional dq-transform detection algorithm is seriously affected by the DDC component.In addition,the proposed improved dq-transform detection algorithm can detect both the steady-state and transient eigenvolumes after the fault.Finally,the proposed algorithm is verified by PSCAD/EMTDC and MATLAB simulation software,and the results show that the proposed algorithm can achieve the detection of both steady-state and transient eigenvolumes.(2)Proposed dq transform detection algorithm for weak fault detection and localizationIn order to realize the detection of weak faults,this dissertation constructs a weak fault detection scheme by combining the weak fault characteristics of two different grounding methods with dq transform detection algorithm.The scheme can realize the detection of single-phase ground fault and early fault of cable in resonant grounding system.To realize fault location,this dissertation proposes a single-phase grounding fault location method based on DDC signal.The method realizes the calculation of circuit impedance by DDC signal parameters,and finally deduces the fault point to busbar distance according to the fault model theory,so as to realize the fault location.The models of single-phase ground fault and cable early fault of resonant grounding system are built by PSCAD/EMTDC software,and the effectiveness of dq transform detection algorithm is verified.