Single-line-to-earth protection on noneffectively-earthed distribution systems through overvoltage- and undervoltage-based philosophies

The effectiveness of overcurrent protection on power systems concerning single-line-to-earth (SLE) faults is strongly related to the adopted neutral-earthing philosophy. Different arrangements of neutral earthing are employed for electric distribution systems (EDSs), being the unearthed, resistance-earthed, inductance-earthed and resonant-earthed designs commonly so-called noneffectively-earthed methods. SLE-fault currents can be very low in these cases, not sensitizing overcurrent-based devices. However, the voltage on the faulted phase decreases, while the voltages on non-faulted phases increase, giving rise to undervoltage and overvoltage conditions. Hence, it is possible not to be restricted to overcurrent-based philosophies but additionally uses overvoltageand undervoltage-based ones. In this sense, this work presents a study of both overvoltage and undervoltage protections combined as a new method for SLEfault protection, aiming the application on noneffectively-earthed EDSs. A relay in ATPDraw is modelled to trip when an undervoltage occurs at one phase and overvoltages occur at the others. ATP-EMTP simulations are performed on a representative EDS, noneffectively-earthed through a neutral earthing resistor (NER), considering different values for the fault resistance and the NER. The results show that the new method is more effective than the overcurrent protection for SLE faults on noneffectively-earthed systems, tripping even when that is not able to do.