Using Discrete Wavelet Transform Technique To Investigate High Impedance Fault and Protection Application Strategy At National Grid

Correct and rapid fault clearing as well as minimizing power outage durations are important utility company concerns. Protections should work properly to detect and isolate faults. However, high-impedance arcing fault, which is not uncommon, poses a challenge for conventional relays due to the low fault levels & complexity. This paper examines the nature of a real-world high impedance arcing fault by using discrete wavelet transform (DWT) method in PSCAD/EMTP software. Based on the analysis results from DWT, the criteria to find the proper relay were established. Then the selected relay was tested by replaying the COMTRADE file captured by station DFR during the fault. On 4/29/2024, an arcing fault occurred on a 69kV line which is protected by a differential & step distance relays. The fault, captured by a street surveillance camara, lasted for approximately 15s and self-extinguished. None of the line protection systems at line terminal stations operated. Thinking about other hi-impedance fault cases in the system, we want to explore more details & nature of the hi-impedance fault. Then the fault information from DFR of this example was reviewed by using DWT tool. DWT methods provide a microscope with multi-resolution properties for closer inspection of transient signals consisting of high frequency oscillations superimposed on 60Hz signal. The presence of 3rd harmonic, accompanied by inter-harmonic, is a good frequency signature to identify high-impedance arcing fault. Together with other requirements, a relay with high-impedance fault detection capability was selected and tested by using the actual fault data. The lab test results were satisfactory. This type of the relay will be installed in MV system for the field pilot operation.