Research On Multi-Level Protection Of Transient Direction Of Small Current Ground Fault

Most of China's medium and low voltage distribution networks use small current grounding systems.Single-phase ground faults occur frequently in small current grounding systems with a probability of more than 80%.Because the distribution network is directly connected to the power users,the structure is complex,the nodes are numerous,and the operation mode is flexible and changeable,which further increases the probability of occurrence of single-phase ground faults in the distribution network.If the wire breaks and falls on the ground,there is still a safety hazard of personal electric shock.With the continuous reform of distribution network operation management mode and the development of small current grounding detection technology,how to quickly and accurately detect and isolate small current ground faults,reduce the scope of fault power outage,restore power supply in time,and improve power supply reliability of distribution network it is of great significance.In recent years,small current ground fault protection technology has been continuously updated,but there are still defects.Therefore,the protection of small current ground faults is still a difficult problem in distribution network research,and further research is needed.Aiming at the shortcomings of the small current ground fault protection technology in the existing distribution network,this paper proposes a multi-level protection technology based on transient direction for small current ground faults,and studies the main factors affecting the multi-level protection technology of transient current direction of small current ground faults.And the corresponding improvement measures have been taken for the fault protection technology.The main research contents are as follows:(1)The grounding method and characteristics of the neutral point of the distribution network are described in detail,and the fault detection and isolation protection are realized by using the transient current and steady-state fault information characteristics of the small current ground fault.The distribution of transient zero-mode current and transient zero-mode voltage in the characteristic frequency band(SFB)of single-phase ground fault in small current grounding system is analyzed.The transient reactive power based on Hilbert transform is used.The fault detection of the power line at the fault line fault point to the bus line section and the fault line downstream section and the sound line direction is implemented,and the feature is used as a protection action criterion for judging the fault line.(2)A multi-level protection technique for small current ground faults using time difference is proposed.First,define the line direction and power parameters respectively,and confirm the ground fault and fault point according to the power supply direction of the distribution line andthe transient reactive power direction;then set the corresponding protection delay value for different distribution lines,and pass the protection delay and The transient reactive power algorithm is matched to achieve self-healing of small current ground faults.(3)Verification using electromagnetic transient simulation and field fault data.The ATP simulation software was used to build a model of multi-level protection technology based on transient direction for small current ground faults.The simulation data was obtained and the data was processed by MATLAB software.The effects of different grounding resistances and different voltage initial phase angles on the protection technique were tested.The results show that the protection technology studied in this paper can effectively detect and isolate ground faults.The multi-level protection technology of small current ground fault based on transient direction proposed in this paper is completed by the feeder terminal alone,and does not depend on the master station and communication.The research results show that the technology can selectively isolate small current ground faults,and compared with the ground-type technology such as voltage-time type,the number of line switch operations is small,fault isolation is fast,and reliability is high.