Small Current Grounding Fault Transient Signal To Indicate The New Method

In China the non-solidly earthed (isolated, high resistance earthed or Petersen coil compensated) neutral is adopted in medium voltage distribution network. The single-phase earth fault has the highest probability in the distribution network, accounting for about 80%.Although the distribution network can last operating for 2hours with the single-phase earth fault, the fault can step further to two or multi-spots fault, thus a power failure occurs. So the fault line should be cut quickly。 It is difficulty to identify the fault signal due to weak current and unstable fault arc. To date utilities still use manual inspection to identify an earthed location in the absence of a more reliable detection method. When single-phase earth fault occurs in the non-solidly earthed neutral system, the transient current of zero mold components is several times or several dozens times of zero sequence current in power frequency. The method based on the transient signals is reliable and sensitive relatively.Firstly Karrenbauer transformation matrix is used to decompose the fault transient signals into independent mold component. From the bus outlet of fault line to the fault point, this article has analysis the phase-frequency characteristics of zero mold and aerial mold impedance in the back of the detection. The conclusion is that, in the selected frequency band, zero mold impedance in the back of the detection is capacitive which is equivalent to a lumped capacitance; aerial mold impedance in the back of the detection is inductive which can be equaled by a lumped inductance. The lump-constant equivalent model is built to represent the relation of any single phase voltage and mold current with this characteristic.Because it is expensive to install three-phase voltage transformer in the open wire, in according of the lump-constant equivalent model, a new fault-indicating principle with installation of only one voltage transformer and three phase current transformer is proposed, and the algorithm is provided. The method adopts the least-square principle to estimate the zero mold equivalent capacitance in the back of the detection with the voltage and current transient information. With the estimation value of zero mold equivalent capacitance the fault indicator can judge whether to giave an alarm or not. For cable, zero sequence voltage transformer an be installed easily, so with the zero mold transient voltage and current the value of zero mold equivalent capacitance can be estimated.The variation characteristics of voltage at the detection points are analyzed in detail in the isolated and Petersen coil compensated neutral system, and the fault phase selection method using one phase voltage variation in power frequency is proposed.The transients produced by an earth fault are simulated using ATP program. The simulation has proved the presented transient signal characteristics and detection methods.The proposed fault indication methods can be used in the non-solidly earthed neural system with higher sensitivity and reliability.