Directional Fault Passage Indicators for Isolated Neutral DistributionGrids

The increasing penetration of Distributed Generation (DG) in the distribution grid that is nowadays taking place challenges the Network Operators to maintain the reliability indexes achieved in prior years. The contribution of the DG-units to the short-circuit currents can cause the mal-operation of devices that are not designed to operate under bi-directional current flows. However, in the isolated-neutral grounding system, in use in countries such as Spain, Finland, China or Japan, the bi-directional fault currents do not affect the performance of fault detection devices in case of phase-to-ground faults, the most common fault type. This is due to the high values of the capacitive, zero-sequence impedance with respect to the other sequence impedances. For this fault type, this earthing system is known as highly reliable because (i) the line-voltages remain unchanged during the fault and (ii) the fault currents are low (difficult to detect), with a high probability of becoming self-extinguishing faults. Because of this, the fault location is mostly based on a trial-and-error switching sequence, until the fault is isolated. This procedure is very time-consuming if performed with manually-operated switches and sectionalizers. Towards reducing the outage time, grid investment is required. One of the options is installing directional Fault Passage Indicators (FPI). This device is already 70 years old, and the technology has already proven to be a cost-effective solution to improve reliability in other grounding systems. Because of the particular features of the isolated-neutral system, directionality is required in any case, with or without DG. Implementing directionality in the conventional way requires installing voltage sensors and the devices are still susceptible to non-detection of high fault impedance faults. The main contributions of the thesis are three different methods to detect the direction in phase-to-ground faults in isolated-neutral grids. The first two methods are voltage sensor-less, whereas the third method is designed for small current fault detection, using zero-sequence voltage and current measurements.status: publishe