A Sensitive Line Differential Protection Solution for High Impedance Fault Detection

Session 6b  Session Title: High impedance fault detection
Chair: Abraham Varghese 

Power systems line faults include not only solid ground faults but also impedance grounded faults such as high impedance grounded faults. It is always required to have proper fault detections for those faults to provide good dependability of fault clearing systems.  Traditionally, line protection systems based on one-end measurements have provided a multiple zones to cover most of solid ground with limited fault resistance coverages and the multiple stages of zero sequence current based protection function is used to detect high impedance faults. The single end solution for the detection of high impedance fault  has both sensitivity limitation and coordination issues due to system source impacts and selectivity demands. 

For line differential protection where a common measuring system is used to detect all types of faults. Detection of high impedance, ground faults may not be possible as it requires lowering the pickup level of line differential protection function to a limit that may affect the scheme security.

This paper has proposed a sensitive line differential protection solution utilizing dedicated measurements based on zero sequence differential current detection with multi-terminal measurement inputs as shown in the figure below. The advantage of using zero sequence line differential is obvious due to the clear selectivity and sensitivity by using differential zero sequence current. Further, with the use of zero sequence-based measurements, the scheme performance is not affected by the presence of the charging current. The proposed function also includes an integrated fast phase selection solution so that the function is suitable for the application where selective single-pole tripping/auto reclosing is required. 

In addition to above features, a three stage of sensitive zero sequence differential current protection has been arranged to hand possible switching impact such as circuit breaker operation in one end of the protected line and shunt reactor operations in the line so that the stability and security of protection solution could be achieved.

The given solution has been verified with simulation model setup in PSCAD environment and the related testing results has approved that proposed sensitive line differential protection function could detect up to 1000 ohms high impedance faults in case of different types of fault conditions such as evolving faults, cross country faults and abnormal conditions such as open current transformer conditions. The paper finally describes the conclusions based on the above tests and verification for different boundary conditions.