Development and test of an adaptive single-pole auto-reclose function

Most faults in solidly grounded high voltage (HV) networks are transient single-phase arcing faults, primarily cleared by single-pole tripping. Yet in many cases a secondary arc is evolving due to the capacitive and inductive coupling from the healthy line phases. This secondary arc eventually extinguishes after some time and auto-reclosing should be executed only after extinction of this secondary arc, because it will be unsuccessful otherwise. This need forces to set long single-pole dead times. A new adaptive auto-reclose function has been developed, which detects the presence and extinction of a secondary arc by monitoring the level of total harmonic distortion (THD) and voltage amplitude at fundamental frequency of the faulty phase voltage (of course this requires line-side voltage transformers (VTs)). Either no arc is present, or the arc is extinguished, if the THD level is small and the fundamental voltage amplitude is reasonably high. If both criteria are fulfilled, the dead time is adapted by the protection relay accordingly. For testing such adaptive auto-reclose functions a simulation-based test, which models a secondary arc as a non-linear element, has been used. The arc is modelled as a simple two-state non-linear fault, which ignites if a certain voltage threshold is exceeded and extinguishes again at the next current zero-crossing. This will result in substantial THD during active arc and clear fundamental frequency voltage as soon as the fault is disappeared. A lot of test cases were executed to cover all the different possible scenarios and timings of the secondary arc behaviour (with and without successful reclosing). Therefore, an iterative closed-loop approach within the simulation-based test software was used to mimic the real-time behaviour of a hardware-in-the-loop test system and simulate correct reaction upon the trip and close commands of the relay under test. The simulated signals are well in line with recordings from real-world events. Such recordings were initially used to validate the function, too, but don't allow full function assessment. Meanwhile first pilot installations of the new function are set up on transmission systems and first field feedback can be presented.