| At present,Neutral ineffectively earthed system is widely used in medium-voltage distribution network in China and some European countries.When a phase to ground fault occurs,the breaker will not be tripped immediately,and it can continue operating for 1～2 hours with faults.But when a phase to phase fault occurs,the breaker is required to cut off the fault line immediately to safeguard the safe operation of the power grid.According to the operation experience of the power grid,most of the faults occurring in the overhead lines are transient,the duration of the faults is very short.The lines return to normal after fault,and the breaker can reclose successfully.But when a fault occurs on a cable line,it is usually a permanent fault.At this time,the state of fault usually lasts for a long time.It is generally not possible to disappear in a short time,and the breaker can’t reclose successfully.In recent years,with the upgrading of domestic distribution network,more and more cables lines are applied in distribution system.The emergence of the mixture of overhead lines and cable lines makes the permanent fault ratio gradually increase.Once the breaker recloses to a permanent fault,it will not only cause the failure of the reclosing,but also cause a strong impact on the system and equipment,making the quality of the grid deteriorated.Therefore,it is important to study a fault detection technique for distribution network which can judge whether there is a fault before the circuit breaker recloses.This paper proposes an active fault detection technique.It can not only be used as fault detection method for de-energized lines,but also can be used as a kind of adaptive reclosing method for distribution lines with longer time limit.The current and voltage signals are quite weak when the breaker trips out in distribution lines.Therefore,the fault detection can be completed by injecting a signal and analyzing the response.The main idea of the method is injecting a voltage pulse signal controlled by a thyristor into the faulted lines before breakers recloses.We can judge whether a fault still exists in the distribution lines by calculating the harmonic impedance of the present system,and calculating the standard deviation of inductance and capacitance under different frequency according to the harmonic impedance.Then the fault detection is completed by calculating the sum of the two standard deviations.Based on the detection result,breakers would receive a signal of reclosing or not by communication.This paper uses theoretical analysis derived the harmonic impedance expressions of transient fault and permanent fault system and calculates the standard deviation of inductance and capacitance under different frequency.After analysis and comparison,we chose the sum of the two standard deviations as criterion.Through simulation experiments and laboratory simulation experiments,we analyze the situation of different triggering angles,different residual capacittance,different fault resistances and different residual loads,and the results verify the effectiveness of the method. |
